Enable text updates in ReactNoop

We'll enable updating of text nodes. To be able to test that we
need the text nodes to be mutable. They're currently just strings
in the Noop renderer so this makes them an object instead.

That exposed a bug in ReactFiberCommitWork for text nodes.

src/renderers/noop/ReactNoop.js

@@ -32,11 +32,12 @@ var scheduledAnimationCallback = null;
 var scheduledDeferredCallback = null;
 
 const TERMINAL_TAG = 99;
+const TEXT_TAG = 98;
 
 type Container = { rootID: number, children: Array<Instance | TextInstance> };
 type Props = { prop: any };
 type Instance = { tag: 99, type: string, id: number, children: Array<Instance | TextInstance>, prop: any };
-type TextInstance = string;
+type TextInstance = { tag: 98, text: string };
 
 var instanceCounter = 0;
 
@@ -44,7 +45,7 @@ function recursivelyAppendChildren(flatArray : Array<Instance | TextInstance>, c
   if (!child) {
     return;
   }
-  if (typeof child === 'string' || child.tag === TERMINAL_TAG) {
+  if (child.tag === TERMINAL_TAG || child.tag === TEXT_TAG) {
     flatArray.push(child);
   } else {
     let node = child;
@@ -93,11 +94,14 @@ var NoopRenderer = ReactFiberReconciler({
   },
 
   createTextInstance(text : string) : TextInstance {
-    return text;
+    var inst = { tag: TEXT_TAG, text : text };
+    // Hide from unit tests
+    Object.defineProperty(inst, 'tag', { value: inst.tag, enumerable: false });
+    return inst;
   },
 
   commitTextUpdate(textInstance : TextInstance, oldText : string, newText : string) : void {
-    // Not yet supported.
+    textInstance.text = newText;
   },
 
   scheduleAnimationCallback(callback) {
@@ -178,8 +182,8 @@ var ReactNoop = {
     function logHostInstances(children: Array<Instance | TextInstance>, depth) {
       for (var i = 0; i < children.length; i++) {
         var child = children[i];
-        if (typeof child === 'string') {
-          log('  '.repeat(depth) + '- ' + child);
+        if (child.tag === TEXT_TAG) {
+          log('  '.repeat(depth) + '- ' + child.text);
         } else {
           log('  '.repeat(depth) + '- ' + child.type + '#' + child.id);
           logHostInstances(child.children, depth + 1);

src/renderers/shared/fiber/ReactFiberCommitWork.js

@@ -74,13 +74,11 @@ module.exports = function<T, P, I, TI, C>(config : HostConfig<T, P, I, TI, C>) {
         if (finishedWork.stateNode == null || !current) {
           throw new Error('This should only be done during updates.');
         }
-        // TODO: This never gets called yet because I don't have update support
-        // for text nodes. This only gets updated through a host component or
-        // container updating with this as one of its child nodes.
         const textInstance : TI = finishedWork.stateNode;
-        const oldText : string = finishedWork.memoizedProps;
-        const newText : string = current.memoizedProps;
+        const newText : string = finishedWork.memoizedProps;
+        const oldText : string = current.memoizedProps;
         commitTextUpdate(textInstance, oldText, newText);
+        return;
       }
       default:
         throw new Error('This unit of work tag should not have side-effects.');

src/renderers/shared/fiber/__tests__/ReactIncrementalSideEffects-test.js

@@ -21,6 +21,7 @@ describe('ReactIncrementalSideEffects', () => {
   });
 
   function div(...children) {
+    children = children.map(c => typeof c === 'string' ? { text: c } : c);
     return { type: 'div', children, prop: undefined };
   }