Implement isMounted for Fiber

This is the naive implementation that doesn't cover the case
where it has completed but not yet committed. It also doesn't
deal with unmounts since they currently don't clean up the item
in the ReactInstanceMap.

src/renderers/shared/fiber/ReactFiberClassComponent.js

@@ -23,6 +23,7 @@ var {
   addCallbackToQueue,
   mergeUpdateQueue,
 } = require('ReactFiberUpdateQueue');
+var { isMounted } = require('ReactFiberTreeReflection');
 var ReactInstanceMap = require('ReactInstanceMap');
 
 module.exports = function(scheduleUpdate : (fiber: Fiber, priorityLevel : PriorityLevel) => void) {
@@ -39,6 +40,7 @@ module.exports = function(scheduleUpdate : (fiber: Fiber, priorityLevel : Priori
 
   // Class component state updater
   const updater = {
+    isMounted,
     enqueueSetState(instance, partialState) {
       const fiber = ReactInstanceMap.get(instance);
       const updateQueue = fiber.updateQueue ?

src/renderers/shared/fiber/ReactFiberTreeReflection.js

@@ -23,6 +23,17 @@ var {
   HostText,
 } = require('ReactTypeOfWork');
 
+exports.isMounted = function(component : ReactComponent<any, any, any>) : boolean {
+  var parent : ?Fiber = ReactInstanceMap.get(component);
+  if (!parent) {
+    return false;
+  }
+  // TODO: This doesn't deal with the case where it has completed but not yet
+  // committed. It also doesn't deal with unmounts since they currently don't
+  // clean up the item in the ReactInstanceMap.
+  return true;
+};
+
 exports.findCurrentHostFiber = function(component : ReactComponent<any, any, any>) : Fiber | null {
   var parent : ?Fiber = ReactInstanceMap.get(component);
   if (!parent) {