Pass prevContext param to componentDidUpdate

This makes use of an expando property, __reactInternalPrevContext, on the stateNode (instance). This resolves the fact that we are not currently passing any value at all to componentDidUpdate for that parameter BUT there still exist some underlying problems with previous context in regard to updates that are aborted before commit.

scripts/fiber/tests-failing.txt

@@ -6,9 +6,6 @@ src/addons/__tests__/ReactFragment-test.js
 * should throw if a plain object even if it is in an owner
 * should throw if a plain object looks like an old element
 
-src/isomorphic/classic/__tests__/ReactContextValidator-test.js
-* should pass previous context to lifecycles
-
 src/renderers/dom/__tests__/ReactDOMProduction-test.js
 * should throw with an error code in production
 

scripts/fiber/tests-passing.txt

@@ -153,6 +153,7 @@ src/isomorphic/children/__tests__/sliceChildren-test.js
 src/isomorphic/classic/__tests__/ReactContextValidator-test.js
 * should filter out context not in contextTypes
 * should pass next context to lifecycles
+* should pass previous context to lifecycles
 * should check context types
 * should check child context types
 

src/renderers/shared/fiber/ReactFiberCommitWork.js

@@ -413,7 +413,8 @@ module.exports = function<T, P, I, TI, C, CX>(
             if (typeof instance.componentDidUpdate === 'function') {
               const prevProps = current.memoizedProps;
               const prevState = current.memoizedState;
-              instance.componentDidUpdate(prevProps, prevState);
+              const prevContext = instance.__reactInternalPrevContext;
+              instance.componentDidUpdate(prevProps, prevState, prevContext);
             }
           }
           attachRef(current, finishedWork, instance);
@@ -422,6 +423,9 @@ module.exports = function<T, P, I, TI, C, CX>(
         if (callbackList) {
           commitCallbacks(finishedWork, callbackList, instance);
         }
+
+        // Store updated context for subsequent calls to componentDidUpdate().
+        instance.__reactInternalPrevContext = instance.context;
         return;
       }
       case HostRoot: {