Make app.dart not spew exceptions on mouse hover

sky/sdk/lib/framework/app.dart

Index: sky/sdk/lib/framework/app.dart
diff --git a/sky/sdk/lib/framework/app.dart b/sky/sdk/lib/framework/app.dart
index 6250fe40d2b9a65b866ae9f2238783a552da1331..7ba95e71b70a99936c8bd32f643220f4b9978992 100644
--- a/sky/sdk/lib/framework/app.dart
+++ b/sky/sdk/lib/framework/app.dart
@@ -14,7 +14,7 @@ class AppView {
     _renderView = new RenderView(child: root);
     _renderView.attach();
     _renderView.layout(newWidth: sky.view.width, newHeight: sky.view.height);
-  
+
     sky.view.scheduleFrame();
   }
 
@@ -51,6 +51,11 @@ class AppView {
         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);