Update implementation-notes.md

prevChild could be CompositeComponent which cause prevChild.node to be undefined.

content/docs/implementation-notes.md

@@ -750,7 +750,7 @@ We collect DOM operations on children in a list so we can execute them in batch:
       // If we can't update an existing instance, we have to unmount it
       // and mount a new one instead of it.
       if (!canUpdate) {
-        var prevNode = prevChild.node;
+        var prevNode = prevChild.getHostNode();
         prevChild.unmount();
 
         var nextChild = instantiateComponent(nextChildren[i]);
@@ -771,7 +771,7 @@ We collect DOM operations on children in a list so we can execute them in batch:
     // Finally, unmount any children that don't exist:
     for (var j = nextChildren.length; j < prevChildren.length; j++) {
      var prevChild = prevRenderedChildren[j];
-     var node = prevChild.node;
+     var node = prevChild.getHostNode();
      prevChild.unmount();
 
      // Record that we need to remove the node