Make app.dart not spew exceptions on mouse hover

sky/sdk/lib/framework/app.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import 'layout2.dart';
 import 'dart:sky' as sky;
 
 class AppView {
 
   AppView(RenderBox root) {
     sky.view.setEventCallback(_handleEvent);
     sky.view.setBeginFrameCallback(_beginFrame);
 
     _renderView = new RenderView(child: root);
     _renderView.attach();
     _renderView.layout(newWidth: sky.view.width, newHeight: sky.view.height);
-  
+
     sky.view.scheduleFrame();
   }
 
   RenderView _renderView;
 
   Map<int, HitTestResult> _hitTestResultForPointer = new Map<int, HitTestResult>();
 
   RenderBox get root => _renderView.child;
   void set root(RenderBox value) {
     _renderView.child = value;
@@ skipping 16 matching lines @@
         _renderView.hitTest(result, x: event.x, y: event.y);
         _hitTestResultForPointer[event.pointer] = result;
         break;
       case 'pointerup':
       case 'pointercancel':
         result = _hitTestResultForPointer[event.pointer];
         _hitTestResultForPointer.remove(event.pointer);
         break;
       case 'pointermove':
         result = _hitTestResultForPointer[event.pointer];
+        // In the case of mouse hover we won't already have a cached down.
+        if (result == null) {
+          result = new HitTestResult();
+          _renderView.hitTest(result, x: event.x, y: event.y);
+        }
         break;
     }
     assert(result != null);
     result.path.reversed.forEach((RenderNode node) {
       node.handlePointer(event);
     });
   }
 
 }