Pass prevContext param to componentDidUpdate

This makes use of an expando property (__reactInternalPrevContext) on the stateNode (instance). A property on the instance was used instead of a Fiber attribute because:
1) Conditional fields on fibers would break monomorphism.
2) Adding to all fibers would bloat types that don't use context.

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, CI>(
             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 +423,12 @@ module.exports = function<T, P, I, TI, C, CX, CI>(
         if (callbackList) {
           commitCallbacks(finishedWork, callbackList, instance);
         }
+
+        // Store updated context for prevContext param in next call to componentDidUpdate().
+        // Use an expando property on instance rather than Fiber because:
+        // 1) Conditional fields on fibers would break monomorphism.
+        // 2) Adding to all fibers would bloat types that don't use context.
+        instance.__reactInternalPrevContext = instance.context;
         return;
       }
       case HostRoot: {