better error message for mergeHelpers and setState

src/core/ReactCompositeComponent.js

@@ -772,6 +772,14 @@ var ReactCompositeComponentMixin = {
       typeof partialState === 'object' || partialState == null,
       'setState(...): takes an object of state variables to update.'
     );
+    if (__DEV__){
+      if (partialState == null) {
+        console.warn(
+          'setState(...): You passed an undefined or null state object; ' +
+          'instead, use forceUpdate().'
+        );
+      }
+    }
     // Merge with `_pendingState` if it exists, otherwise with existing state.
     this.replaceState(
       merge(this._pendingState || this.state, partialState),

src/vendor/core/mergeHelpers.js

@@ -66,9 +66,9 @@ var mergeHelpers = {
   checkMergeArrayArgs: function(one, two) {
     invariant(
       Array.isArray(one) && Array.isArray(two),
-      'Critical assumptions about the merge functions have been violated. ' +
-      'This is the fault of the merge functions themselves, not necessarily ' +
-      'the callers.'
+      'Tried to merge arrays, instead got %s and %s.',
+      one,
+      two
     );
   },
 
@@ -87,9 +87,8 @@ var mergeHelpers = {
   checkMergeObjectArg: function(arg) {
     invariant(
       !isTerminal(arg) && !Array.isArray(arg),
-      'Critical assumptions about the merge functions have been violated. ' +
-      'This is the fault of the merge functions themselves, not necessarily ' +
-      'the callers.'
+      'Tried to merge an object, instead got %s.',
+      arg
     );
   },