Update shallowCompare to accept nextContext (#6661)

* Update shallowCompare to accept nextContext

Across our application we are using immutable objects as properties and thus using shallowCompare for all our {{shouldComponentUpdate}}.  Because of this children with contextTypes don't get updates when the context on the parent changes.  Adding an additional comparison for context (when it is provided) fixes this problem.

* Remove the undefined check

* Add nextContext

src/addons/ReactComponentWithPureRenderMixin.js

@@ -40,8 +40,8 @@ var shallowCompare = require('shallowCompare');
  * See https://facebook.github.io/react/docs/pure-render-mixin.html
  */
 var ReactComponentWithPureRenderMixin = {
-  shouldComponentUpdate: function(nextProps, nextState) {
-    return shallowCompare(this, nextProps, nextState);
+  shouldComponentUpdate: function(nextProps, nextState, nextContext) {
+    return shallowCompare(this, nextProps, nextState, nextContext);
   },
 };
 

src/addons/shallowCompare.js

@@ -18,10 +18,11 @@ var shallowEqual = require('shallowEqual');
  * See ReactComponentWithPureRenderMixin
  * See also https://facebook.github.io/react/docs/shallow-compare.html
  */
-function shallowCompare(instance, nextProps, nextState) {
+function shallowCompare(instance, nextProps, nextState, nextContext) {
   return (
     !shallowEqual(instance.props, nextProps) ||
-    !shallowEqual(instance.state, nextState)
+    !shallowEqual(instance.state, nextState) ||
+    !shallowEqual(instance.context, nextContext)
   );
 }