Begin work on the PopupMenu entrance animation

sky/framework/fn.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.
 
 library fn;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:sky' as sky;
 import 'reflect.dart' as reflect;
@@ skipping 661 matching lines @@
 
     var vdom = _vdom;
     while (vdom is Component) {
       vdom = vdom._vdom;
     }
 
     assert(vdom._root != null);
     _syncInternal();
   }
 
+  void scheduleBuild() {
+    setState(() {});
+  }
+
   void setState(Function fn()) {
     assert(_vdom != null || _defunct); // cannot setState before mounting.
     _stateful = true;
     fn();
     if (!_defunct && _currentlyRendering != this) {
       _dirty = true;
       _scheduleComponentForRender(this);
     }
   }
 
@@ skipping 11 matching lines @@
 
       _mount(null, _host, null);
       assert(_root is sky.Node);
 
       sw.stop();
       if (_shouldLogRenderDuration)
         print("Initial build: ${sw.elapsedMicroseconds} microseconds");
     });
   }
 }