Downgrade deprecation warnings from errors to warnings (#9650)

* Initial regeneration of results.json

**what is the change?:**
We ran `yarn build` and updated the perf. stats record.

**why make this change?:**
Some commits have landed without updating this. By getting an initial update, I can run the build script again after my changes and see any size regressions.

* Downgrade deprecation warnings from errors to warnings

**what is the change?:**
Swapping out `warning` module for a fork that uses `console.warn`.
It looks like we were using the `warning` module for deprecation notices, *but* there is also a 'deprecated' module designed specifically for deprecation notices.

However, we could not find any place that it was currently used.

Since React's build process is not 100% clear to me, I assume it could still be used somewhere by something and just updated it along with other deprecation notices.

We might consider a follow-up diff that does some clean up here;
 - remove 'deprecated' module if it's unused, OR
 - use 'deprecated' module for all our current deprecation warnings

**why make this change?:**
- We have had complaints about noisy warnings, in particular after introducing new deprecations
- They potentially cause CI failures
- Deprecations are not really time-sensitive, can ship without breaking your app, etc.

For more context - https://github.com/facebook/react/issues/9395

**test plan:**
`npm run test`
and unit tests for the new modules
and manual testing (WIP)

**issue:**
https://github.com/facebook/react/issues/9395

* Add 'lowPriorityWarning' to ReactExternals

**what is the change?:**
We won't bundle 'lowPriorityWarning' with the rest of React when building for Facebook.
NOTE: A parallel commit will introduce an internal implementation of 'lowPriorityWarning' in Facebook's codebase, to compensate. Will post a comment with the diff number once that is up.

**why make this change?:**
So that the sync between github and Facebook can go more smoothly!

**test plan:**
We will see when I run the sync! But this is a reasonable first step imo.

**issue:**
https://github.com/facebook/react/issues/9398

* Make state mutations an error, not low-pri warning

**what is the change?:**
Even though this is a "deprecation" warning, we still want to use 'console.error' for it.

**why make this change?:**
- It's not likely to come up now, hopefully, because this warning has been present for some time
- This will cause real issues in production if ignored

**test plan:**
`yarn test` - we did fix one test which failed bc of this change

**issue:**
https://github.com/facebook/react/issues/9398

* Fix test of assigning to this.state that was only passing in fiber

**what is the change?:**
updated a unit test for assigning directly to state; it once again raises an error and not a warning.

**why make this change?:**
So that tests pass

**test plan:**
 REACT_DOM_JEST_USE_FIBER=1 yarn run test

**issue:**

* Update results.json

scripts/rollup/modules.js

@@ -162,6 +162,7 @@ function getExternalModules(externals, bundleType, isRenderer) {
     case FB_PROD:
       fbjsModules.forEach(module => externalModules.push(module));
       externalModules.push('ReactCurrentOwner');
+      externalModules.push('lowPriorityWarning');
       if (isRenderer) {
         externalModules.push('React');
         if (externalModules.indexOf('react-dom') > -1) {

scripts/rollup/results.json

@@ -1,92 +1,92 @@
 {
   "bundleSizes": {
     "react.development.js (UMD_DEV)": {
-      "size": 123496,
-      "gzip": 31034
+      "size": 125119,
+      "gzip": 31339
     },
     "react.production.min.js (UMD_PROD)": {
       "size": 15753,
       "gzip": 5824
     },
     "react-dom.development.js (UMD_DEV)": {
-      "size": 592317,
-      "gzip": 136560
+      "size": 593408,
+      "gzip": 136701
     },
     "react-dom.production.min.js (UMD_PROD)": {
-      "size": 121979,
-      "gzip": 38564
+      "size": 122214,
+      "gzip": 38682
     },
     "react-dom-server.development.js (UMD_DEV)": {
-      "size": 498439,
-      "gzip": 120624
+      "size": 525896,
+      "gzip": 126687
     },
     "react-dom-server.production.min.js (UMD_PROD)": {
-      "size": 107169,
-      "gzip": 33472
+      "size": 111581,
+      "gzip": 34959
     },
     "react-art.development.js (UMD_DEV)": {
-      "size": 349177,
-      "gzip": 77902
+      "size": 349191,
+      "gzip": 77905
     },
     "react-art.production.min.js (UMD_PROD)": {
-      "size": 96267,
-      "gzip": 29310
+      "size": 96277,
+      "gzip": 29313
     },
     "react.development.js (NODE_DEV)": {
-      "size": 70293,
-      "gzip": 17777
+      "size": 71914,
+      "gzip": 18276
     },
     "react.production.min.js (NODE_PROD)": {
       "size": 9195,
       "gzip": 3614
     },
     "React-dev.js (FB_DEV)": {
-      "size": 71576,
-      "gzip": 18181
+      "size": 73423,
+      "gzip": 18751
     },
     "React-prod.js (FB_PROD)": {
-      "size": 36456,
-      "gzip": 9216
+      "size": 36836,
+      "gzip": 9248
     },
     "ReactDOMStack-dev.js (FB_DEV)": {
-      "size": 494069,
-      "gzip": 117876
+      "size": 496986,
+      "gzip": 118763
     },
     "ReactDOMStack-prod.js (FB_PROD)": {
-      "size": 352489,
-      "gzip": 84575
+      "size": 353129,
+      "gzip": 84773
     },
     "react-dom.development.js (NODE_DEV)": {
-      "size": 549893,
-      "gzip": 126464
+      "size": 550984,
+      "gzip": 126642
     },
     "react-dom.production.min.js (NODE_PROD)": {
-      "size": 118192,
-      "gzip": 37224
+      "size": 118427,
+      "gzip": 37300
     },
     "ReactDOMFiber-dev.js (FB_DEV)": {
-      "size": 550815,
-      "gzip": 126958
+      "size": 551962,
+      "gzip": 127141
     },
     "ReactDOMFiber-prod.js (FB_PROD)": {
-      "size": 410570,
-      "gzip": 94005
+      "size": 411657,
+      "gzip": 94084
     },
     "react-dom-server.development.js (NODE_DEV)": {
-      "size": 447050,
-      "gzip": 108025
+      "size": 474459,
+      "gzip": 114087
     },
     "react-dom-server.production.min.js (NODE_PROD)": {
-      "size": 101542,
-      "gzip": 31424
+      "size": 105956,
+      "gzip": 32782
     },
     "ReactDOMServerStack-dev.js (FB_DEV)": {
-      "size": 445736,
-      "gzip": 107878
+      "size": 455985,
+      "gzip": 109831
     },
     "ReactDOMServerStack-prod.js (FB_PROD)": {
-      "size": 333450,
-      "gzip": 80260
+      "size": 334090,
+      "gzip": 80459
     },
     "ReactARTStack-dev.js (FB_DEV)": {
       "size": 143166,
@@ -97,20 +97,20 @@
       "gzip": 23039
     },
     "react-art.development.js (NODE_DEV)": {
-      "size": 270501,
-      "gzip": 57767
+      "size": 270515,
+      "gzip": 57772
     },
     "react-art.production.min.js (NODE_PROD)": {
-      "size": 57652,
-      "gzip": 17386
+      "size": 57662,
+      "gzip": 17391
     },
     "ReactARTFiber-dev.js (FB_DEV)": {
-      "size": 269759,
-      "gzip": 57590
+      "size": 269773,
+      "gzip": 57593
     },
     "ReactARTFiber-prod.js (FB_PROD)": {
-      "size": 208039,
-      "gzip": 43486
+      "size": 208053,
+      "gzip": 43488
     },
     "ReactNativeStack.js (RN)": {
       "size": 233993,
@@ -121,20 +121,20 @@
       "gzip": 84001
     },
     "ReactTestRendererFiber-dev.js (FB_DEV)": {
-      "size": 267261,
-      "gzip": 56462
+      "size": 267275,
+      "gzip": 56465
     },
     "ReactTestRendererStack-dev.js (FB_DEV)": {
       "size": 151770,
       "gzip": 34846
     },
     "react-noop-renderer.development.js (NODE_DEV)": {
-      "size": 259178,
-      "gzip": 54431
+      "size": 259192,
+      "gzip": 54435
     },
     "react-test-renderer.development.js (NODE_DEV)": {
-      "size": 268012,
-      "gzip": 56651
+      "size": 268026,
+      "gzip": 56654
     },
     "react-dom-test-utils.development.js (NODE_DEV)": {
       "size": 52792,
@@ -155,6 +155,14 @@
     "ReactShallowRenderer-dev.js (FB_DEV)": {
       "size": 8063,
       "gzip": 2229
+    },
+    "ReactDOMServerStream-dev.js (FB_DEV)": {
+      "size": 472917,
+      "gzip": 113925
+    },
+    "ReactDOMServerStream-prod.js (FB_PROD)": {
+      "size": 345920,
+      "gzip": 83497
     }
   }
 }

src/addons/__tests__/ReactDOMFactories-test.js

@@ -17,13 +17,15 @@ var {div} = require('ReactDOMFactories');
 describe('ReactDOMFactories', () => {
   it('allow factories to be called without warnings', () => {
     spyOn(console, 'error');
+    spyOn(console, 'warn');
     var element = div();
     expect(element.type).toBe('div');
     expect(console.error).not.toHaveBeenCalled();
+    expect(console.warn).not.toHaveBeenCalled();
   });
 
   it('warns once when accessing React.DOM methods', () => {
-    spyOn(console, 'error');
+    spyOn(console, 'warn');
 
     var a = React.DOM.a();
     var p = React.DOM.p();
@@ -31,8 +33,8 @@ describe('ReactDOMFactories', () => {
     expect(a.type).toBe('a');
     expect(p.type).toBe('p');
 
-    expect(console.error).toHaveBeenCalledTimes(1);
-    expect(console.error.calls.first().args[0]).toContain(
+    expect(console.warn).toHaveBeenCalledTimes(1);
+    expect(console.warn.calls.first().args[0]).toContain(
       'Warning: Accessing factories like React.DOM.a has been deprecated',
     );
   });

src/isomorphic/React.js

@@ -27,7 +27,7 @@ var createFactory = ReactElement.createFactory;
 var cloneElement = ReactElement.cloneElement;
 
 if (__DEV__) {
-  var warning = require('fbjs/lib/warning');
+  var lowPriorityWarning = require('lowPriorityWarning');
   var canDefineProperty = require('canDefineProperty');
   var ReactElementValidator = require('ReactElementValidator');
   createElement = ReactElementValidator.createElement;
@@ -91,7 +91,7 @@ if (__DEV__) {
   let warnedForPropTypes = false;
 
   React.createMixin = function(mixin) {
-    warning(
+    lowPriorityWarning(
       warnedForCreateMixin,
       'React.createMixin is deprecated and should not be used. You ' +
         'can use this mixin directly instead.',
@@ -104,7 +104,7 @@ if (__DEV__) {
   if (canDefineProperty) {
     Object.defineProperty(React, 'checkPropTypes', {
       get() {
-        warning(
+        lowPriorityWarning(
           warnedForCheckPropTypes,
           'checkPropTypes has been moved to a separate package. ' +
             'Accessing React.checkPropTypes is no longer supported ' +
@@ -119,7 +119,7 @@ if (__DEV__) {
 
     Object.defineProperty(React, 'createClass', {
       get: function() {
-        warning(
+        lowPriorityWarning(
           warnedForCreateClass,
           'React.createClass is no longer supported. Use a plain JavaScript ' +
             "class instead. If you're not yet ready to migrate, " +
@@ -133,7 +133,7 @@ if (__DEV__) {
 
     Object.defineProperty(React, 'PropTypes', {
       get() {
-        warning(
+        lowPriorityWarning(
           warnedForPropTypes,
           'PropTypes has been moved to a separate package. ' +
             'Accessing React.PropTypes is no longer supported ' +
@@ -155,7 +155,7 @@ if (__DEV__) {
   Object.keys(ReactDOMFactories).forEach(function(factory) {
     React.DOM[factory] = function(...args) {
       if (!warnedForFactories) {
-        warning(
+        lowPriorityWarning(
           false,
           'Accessing factories like React.DOM.%s has been deprecated ' +
             'and will be removed in the future. Use the ' +

src/isomorphic/__tests__/React-test.js

@@ -19,22 +19,22 @@ describe('React', () => {
   });
 
   it('should log a deprecation warning once when using React.createMixin', () => {
-    spyOn(console, 'error');
+    spyOn(console, 'warn');
     React.createMixin();
     React.createMixin();
-    expectDev(console.error.calls.count()).toBe(1);
-    expectDev(console.error.calls.argsFor(0)[0]).toContain(
+    expectDev(console.warn.calls.count()).toBe(1);
+    expectDev(console.warn.calls.argsFor(0)[0]).toContain(
       'React.createMixin is deprecated and should not be used',
     );
   });
 
   it('should warn once when attempting to access React.createClass', () => {
-    spyOn(console, 'error');
+    spyOn(console, 'warn');
     let createClass = React.createClass;
     createClass = React.createClass;
     expect(createClass).not.toBe(undefined);
-    expectDev(console.error.calls.count()).toBe(1);
-    expectDev(console.error.calls.argsFor(0)[0]).toContain(
+    expectDev(console.warn.calls.count()).toBe(1);
+    expectDev(console.warn.calls.argsFor(0)[0]).toContain(
       'React.createClass is no longer supported. Use a plain JavaScript ' +
         "class instead. If you're not yet ready to migrate, " +
         'create-react-class is available on npm as a drop-in replacement. ' +
@@ -43,12 +43,12 @@ describe('React', () => {
   });
 
   it('should warn once when attempting to access React.PropTypes', () => {
-    spyOn(console, 'error');
+    spyOn(console, 'warn');
     let PropTypes = React.PropTypes;
     PropTypes = React.PropTypes;
     expect(PropTypes).not.toBe(undefined);
-    expectDev(console.error.calls.count()).toBe(1);
-    expectDev(console.error.calls.argsFor(0)[0]).toContain(
+    expectDev(console.warn.calls.count()).toBe(1);
+    expectDev(console.warn.calls.argsFor(0)[0]).toContain(
       'PropTypes has been moved to a separate package. ' +
         'Accessing React.PropTypes is no longer supported ' +
         'and will be removed completely in React 16. ' +
@@ -58,12 +58,12 @@ describe('React', () => {
   });
 
   it('should warn once when attempting to access React.checkPropTypes', () => {
-    spyOn(console, 'error');
+    spyOn(console, 'warn');
     let checkPropTypes = React.checkPropTypes;
     checkPropTypes = React.checkPropTypes;
     expect(checkPropTypes).not.toBe(undefined);
-    expectDev(console.error.calls.count()).toBe(1);
-    expectDev(console.error.calls.argsFor(0)[0]).toContain(
+    expectDev(console.warn.calls.count()).toBe(1);
+    expectDev(console.warn.calls.argsFor(0)[0]).toContain(
       'checkPropTypes has been moved to a separate package. ' +
         'Accessing React.checkPropTypes is no longer supported ' +
         'and will be removed completely in React 16. ' +

src/isomorphic/classic/element/ReactElementValidator.js

@@ -27,8 +27,9 @@ var getIteratorFn = require('getIteratorFn');
 
 if (__DEV__) {
   var checkPropTypes = require('prop-types/checkPropTypes');
-  var warning = require('fbjs/lib/warning');
+  var lowPriorityWarning = require('lowPriorityWarning');
   var ReactDebugCurrentFrame = require('ReactDebugCurrentFrame');
+  var warning = require('fbjs/lib/warning');
   var {getCurrentStackAddendum} = require('ReactComponentTreeHook');
 }
 
@@ -285,7 +286,7 @@ var ReactElementValidator = {
         Object.defineProperty(validatedFactory, 'type', {
           enumerable: false,
           get: function() {
-            warning(
+            lowPriorityWarning(
               false,
               'Factory.type is deprecated. Access the class directly ' +
                 'before passing it to createFactory.',

src/isomorphic/classic/element/__tests__/ReactElementValidator-test.js

@@ -441,20 +441,20 @@ describe('ReactElementValidator', () => {
   });
 
   it('should warn when accessing .type on an element factory', () => {
-    spyOn(console, 'error');
+    spyOn(console, 'warn');
     function TestComponent() {
       return <div />;
     }
     var TestFactory = React.createFactory(TestComponent);
     expect(TestFactory.type).toBe(TestComponent);
-    expectDev(console.error.calls.count()).toBe(1);
-    expectDev(console.error.calls.argsFor(0)[0]).toBe(
+    expectDev(console.warn.calls.count()).toBe(1);
+    expectDev(console.warn.calls.argsFor(0)[0]).toBe(
       'Warning: Factory.type is deprecated. Access the class directly before ' +
         'passing it to createFactory.',
     );
     // Warn once, not again
     expect(TestFactory.type).toBe(TestComponent);
-    expectDev(console.error.calls.count()).toBe(1);
+    expectDev(console.warn.calls.count()).toBe(1);
   });
 
   it('does not warn when using DOM node as children', () => {

src/isomorphic/modern/class/ReactBaseClasses.js

@@ -16,7 +16,7 @@ var ReactNoopUpdateQueue = require('ReactNoopUpdateQueue');
 var canDefineProperty = require('canDefineProperty');
 var emptyObject = require('fbjs/lib/emptyObject');
 var invariant = require('fbjs/lib/invariant');
-var warning = require('fbjs/lib/warning');
+var lowPriorityWarning = require('lowPriorityWarning');
 
 /**
  * Base class helpers for the updating state of a component.
@@ -108,7 +108,7 @@ if (__DEV__) {
     if (canDefineProperty) {
       Object.defineProperty(ReactComponent.prototype, methodName, {
         get: function() {
-          warning(
+          lowPriorityWarning(
             false,
             '%s(...) is deprecated in plain JavaScript React classes. %s',
             info[0],

src/isomorphic/modern/class/__tests__/ReactCoffeeScriptClass-test.coffee

@@ -378,16 +378,16 @@ describe 'ReactCoffeeScriptClass', ->
     undefined
 
   it 'should throw AND warn when trying to access classic APIs', ->
-    spyOn console, 'error'
+    spyOn console, 'warn'
     instance =
       test Inner(name: 'foo'), 'DIV', 'foo'
     expect(-> instance.replaceState {}).toThrow()
     expect(-> instance.isMounted()).toThrow()
-    expect(console.error.calls.count()).toBe 2
-    expect(console.error.calls.argsFor(0)[0]).toContain(
+    expect(console.warn.calls.count()).toBe 2
+    expect(console.warn.calls.argsFor(0)[0]).toContain(
       'replaceState(...) is deprecated in plain JavaScript React classes'
     )
-    expect(console.error.calls.argsFor(1)[0]).toContain(
+    expect(console.warn.calls.argsFor(1)[0]).toContain(
       'isMounted(...) is deprecated in plain JavaScript React classes'
     )
     undefined

src/isomorphic/modern/class/__tests__/ReactES6Class-test.js

@@ -402,15 +402,15 @@ describe('ReactES6Class', () => {
   });
 
   it('should throw AND warn when trying to access classic APIs', () => {
-    spyOn(console, 'error');
+    spyOn(console, 'warn');
     var instance = test(<Inner name="foo" />, 'DIV', 'foo');
     expect(() => instance.replaceState({})).toThrow();
     expect(() => instance.isMounted()).toThrow();
-    expect(console.error.calls.count()).toBe(2);
-    expect(console.error.calls.argsFor(0)[0]).toContain(
+    expect(console.warn.calls.count()).toBe(2);
+    expect(console.warn.calls.argsFor(0)[0]).toContain(
       'replaceState(...) is deprecated in plain JavaScript React classes',
     );
-    expect(console.error.calls.argsFor(1)[0]).toContain(
+    expect(console.warn.calls.argsFor(1)[0]).toContain(
       'isMounted(...) is deprecated in plain JavaScript React classes',
     );
   });

src/isomorphic/modern/class/__tests__/ReactTypeScriptClass-test.ts

@@ -502,18 +502,18 @@ describe('ReactTypeScriptClass', function() {
   });
 
   it('should throw AND warn when trying to access classic APIs', function() {
-    spyOn(console, 'error');
+    spyOn(console, 'warn');
     var instance = test(
       React.createElement(Inner, {name: 'foo'}),
       'DIV','foo'
     );
     expect(() => instance.replaceState({})).toThrow();
     expect(() => instance.isMounted()).toThrow();
-    expect((<any>console.error).calls.count()).toBe(2);
-    expect((<any>console.error).calls.argsFor(0)[0]).toContain(
+    expect((<any>console.warn).calls.count()).toBe(2);
+    expect((<any>console.warn).calls.argsFor(0)[0]).toContain(
       'replaceState(...) is deprecated in plain JavaScript React classes'
     );
-    expect((<any>console.error).calls.argsFor(1)[0]).toContain(
+    expect((<any>console.warn).calls.argsFor(1)[0]).toContain(
       'isMounted(...) is deprecated in plain JavaScript React classes'
     );
   });

src/renderers/__tests__/ReactPerf-test.js

@@ -406,30 +406,30 @@ describeStack('ReactPerf', () => {
 
   it('warns once when using getMeasurementsSummaryMap', () => {
     var measurements = measure(() => {});
-    spyOn(console, 'error');
+    spyOn(console, 'warn');
     ReactPerf.getMeasurementsSummaryMap(measurements);
-    expectDev(console.error.calls.count()).toBe(1);
-    expectDev(console.error.calls.argsFor(0)[0]).toContain(
+    expectDev(console.warn.calls.count()).toBe(1);
+    expectDev(console.warn.calls.argsFor(0)[0]).toContain(
       '`ReactPerf.getMeasurementsSummaryMap(...)` is deprecated. Use ' +
         '`ReactPerf.getWasted(...)` instead.',
     );
 
     ReactPerf.getMeasurementsSummaryMap(measurements);
-    expectDev(console.error.calls.count()).toBe(1);
+    expectDev(console.warn.calls.count()).toBe(1);
   });
 
   it('warns once when using printDOM', () => {
     var measurements = measure(() => {});
-    spyOn(console, 'error');
+    spyOn(console, 'warn');
     ReactPerf.printDOM(measurements);
-    expectDev(console.error.calls.count()).toBe(1);
-    expectDev(console.error.calls.argsFor(0)[0]).toContain(
+    expectDev(console.warn.calls.count()).toBe(1);
+    expectDev(console.warn.calls.argsFor(0)[0]).toContain(
       '`ReactPerf.printDOM(...)` is deprecated. Use ' +
         '`ReactPerf.printOperations(...)` instead.',
     );
 
     ReactPerf.printDOM(measurements);
-    expectDev(console.error.calls.count()).toBe(1);
+    expectDev(console.warn.calls.count()).toBe(1);
   });
 
   it('returns isRunning state', () => {

src/renderers/shared/ReactPerf.js

@@ -13,7 +13,7 @@
 'use strict';
 
 var ReactDebugTool = require('ReactDebugTool');
-var warning = require('fbjs/lib/warning');
+var lowPriorityWarning = require('lowPriorityWarning');
 var alreadyWarned = false;
 
 import type {FlushHistory} from 'ReactDebugTool';
@@ -390,7 +390,7 @@ function printOperations(flushHistory?: FlushHistory) {
 
 var warnedAboutPrintDOM = false;
 function printDOM(measurements: FlushHistory) {
-  warning(
+  lowPriorityWarning(
     warnedAboutPrintDOM,
     '`ReactPerf.printDOM(...)` is deprecated. Use ' +
       '`ReactPerf.printOperations(...)` instead.',
@@ -401,7 +401,7 @@ function printDOM(measurements: FlushHistory) {
 
 var warnedAboutGetMeasurementsSummaryMap = false;
 function getMeasurementsSummaryMap(measurements: FlushHistory) {
-  warning(
+  lowPriorityWarning(
     warnedAboutGetMeasurementsSummaryMap,
     '`ReactPerf.getMeasurementsSummaryMap(...)` is deprecated. Use ' +
       '`ReactPerf.getWasted(...)` instead.',

src/shared/utils/deprecated.js

@@ -12,7 +12,7 @@
 
 'use strict';
 
-var warning = require('fbjs/lib/warning');
+var lowPriorityWarning = require('lowPriorityWarning');
 
 /**
  * This will log a single deprecation notice per function and forward the call
@@ -35,7 +35,7 @@ function deprecated<T: Function>(
   var warned = false;
   if (__DEV__) {
     var newFn = function() {
-      warning(
+      lowPriorityWarning(
         warned,
         /* eslint-disable no-useless-concat */
         // Require examples in this string must be split to prevent React's

src/shared/utils/lowPriorityWarning.js

@@ -0,0 +1,41 @@
+/**
+ * Copyright 2014-2015, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @providesModule lowPriorityWarning
+ */
+
+'use strict';
+
+/**
+ * Forked from fbjs/warning:
+ * https://github.com/facebook/fbjs/blob/e66ba20ad5be433eb54423f2b097d829324d9de6/packages/fbjs/src/__forks__/warning.js
+ *
+ * Only change is we use console.warn instead of console.error,
+ * and do nothing when 'console' is not supported.
+ * This really simplifies the code.
+ * ---
+ *
+ * Similar to invariant but only logs a warning if the condition is not met.
+ * This can be used to log issues in development environments in critical
+ * paths. Removing the logging code for production environments will keep the
+ * same logic and follow the same code paths.
+ */
+
+var lowPriorityWarning = function() {};
+
+if (__DEV__) {
+  lowPriorityWarning = function(condition, format, ...args) {
+    var argIndex = 0;
+    var message = 'Warning: ' + format.replace(/%s/g, () => args[argIndex++]);
+    if (!condition && typeof console !== 'undefined') {
+      console.warn(message);
+    }
+  };
+}
+
+module.exports = lowPriorityWarning;