Begin work on the PopupMenu entrance animation

sky/framework/animation/animation.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 'curves.dart';
-import 'dart:async';
-import 'generators.dart';
-
-class Animation {
-  Stream<double> get onValueChanged => _controller.stream;
-
-  double get value => _value;
-
-  void set value(double value) {
-    stop();
-   _setValue(value);
-  }
-
-  bool get isAnimating => _animation != null;
-
-  StreamController _controller = new StreamController(sync: true);
-
-  AnimationGenerator _animation;
-
-  double _value;
-
-  void _setValue(double value) {
-    _value = value;
-    _controller.add(_value);
-  }
-
-  void stop() {
-    if (_animation != null) {
-      _animation.cancel();
-      _animation = null;
-    }
-  }
-
-  void animateTo(double newValue, double duration,
-      { Curve curve: linear, double initialDelay: 0.0 }) {
-    stop();
-
-    _animation = new AnimationGenerator(
-        duration: duration,
-        begin: _value,
-        end: newValue,
-        curve: curve,
-        initialDelay: initialDelay);
-
-    _animation.onTick.listen(_setValue, onDone: () {
-      _animation = null;
-    });
-  }
-}