Make app.dart not spew exceptions on mouse hover

app.dart assumes touch devices, which is great.  We
just don't need to spam exceptions when testing inside
mojo_shell with a mouse. :)

R=jackson@google.com, ianh@google.com

Review URL: https://codereview.chromium.org/1160853003

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);