Include Object.assign polyfill

This isn't a good final solution but it makes React actually usable on
its own.

This also makes tests runnable, though only via jest
(./node_modules/.bin/jest)

jest/environment.js

@@ -1 +1,3 @@
 __DEV__ = true;
+
+require.requireActual('../src/vendor/polyfill/Object.es6.js');

src/browser/ui/React.js

@@ -18,6 +18,10 @@
 
 "use strict";
 
+// TODO: Move this elsewhere - it only exists in open source until a better
+// solution is found.
+require('Object.es6');
+
 var DOMPropertyOperations = require('DOMPropertyOperations');
 var EventPluginUtils = require('EventPluginUtils');
 var ReactChildren = require('ReactChildren');

src/vendor/polyfill/Object.es6.js

@@ -0,0 +1,53 @@
+/**
+ * Copyright 2014 Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * @providesModule Object.es6
+ * @polyfill
+ */
+
+// https://people.mozilla.org/~jorendorff/es6-draft.html#sec-object.assign
+
+if (!Object.assign) {
+  Object.assign = function(target, sources) {
+    if (target == null) {
+      throw new TypeError('Object.assign target cannot be null or undefined');
+    }
+    var to = Object(target);
+
+    var hasOwnProperty = Object.prototype.hasOwnProperty;
+
+    for (var nextIndex = 1; nextIndex < arguments.length; nextIndex++) {
+      var nextSource = arguments[nextIndex];
+      if (nextSource == null) {
+        throw new TypeError('Object.assign source cannot be null or undefined');
+      }
+
+      var from = Object(nextSource);
+
+      // We don't currently support accessors nor proxies. Therefore this copy
+      // cannot throw. If we ever supported this then we must handle
+      // exceptions and side-effects.
+
+      for (var key in from) {
+        if (hasOwnProperty.call(from, key)) {
+          to[key] = from[key];
+        }
+      }
+    }
+
+    return to;
+  };
+}
+