Fix findComponentRoot w/ Unidentified Nodes

The current `ReactInstanceHandles` has a bug where `findComponentRoot` barfs if it comes across a node that was not identified by React (via `ReactID`). This fixes that.

This was always a bug, but it became more apparent once we switched to `data-reactid` because arbitrary `document.createElement`'d nodes are much more likely to have an `id` than they are to have a `data-reactid`.

src/core/ReactInstanceHandles.js

@@ -286,10 +286,10 @@ var ReactInstanceHandles = {
   findComponentRoot: function(ancestorNode, id) {
     var child = ancestorNode.firstChild;
     while (child) {
-      var childId = ReactID.getID(child);
-      if (id === childId) {
+      var childID = ReactID.getID(child);
+      if (id === childID) {
         return child;
-      } else if (isAncestorIDOf(childId, id)) {
+      } else if (childID && isAncestorIDOf(childID, id)) {
         return ReactInstanceHandles.findComponentRoot(child, id);
       }
       child = child.nextSibling;

src/core/__tests__/ReactInstanceHandles-test.js

@@ -102,6 +102,25 @@ describe('ReactInstanceHandles', function() {
         )
       ).toBe(childNodeB);
     });
+
+    it('should work around unidentified nodes', function() {
+      var parentNode = document.createElement('div');
+      var childNodeA = document.createElement('div');
+      var childNodeB = document.createElement('div');
+      parentNode.appendChild(childNodeA);
+      parentNode.appendChild(childNodeB);
+
+      ReactID.setID(parentNode, '.react[0]');
+      // No ID on `childNodeA`.
+      ReactID.setID(childNodeB, '.react[0].0:1');
+
+      expect(
+        ReactInstanceHandles.findComponentRoot(
+          parentNode,
+          ReactID.getID(childNodeB)
+        )
+      ).toBe(childNodeB);
+    });
   });
 
   describe('getReactRootIDFromNodeID', function() {