Simplify the Drawer animation

sky/sdk/lib/widgets/animated.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 'package:sky/animation/animation_performance.dart';
+import 'package:sky/widgets/animated_component.dart';
+import 'package:sky/widgets/animation_builder.dart';
+import 'package:sky/widgets/basic.dart';
+import 'package:vector_math/vector_math.dart';
+
+class PositionAnimation extends AnimatedComponent {

[Matt Perry, 2015/07/15 00:20:18]

nit: I like AnimatedFoo convention for animated components

+  PositionAnimation({
+    String key,
+    this.child,
+    this.performance,
+    this.position
+  }) : super(key: key);
+
+  Widget child;
+  AnimatedType<Point> position;
+  AnimationPerformance performance;
+
+  void initState() {
+    watch(performance);
+  }
+
+  void syncFields(PositionAnimation source) {
+    child = source.child;
+    position = source.position;
+    assert(performance == source.performance);
+  }
+
+  Widget build() {
+    Matrix4 transform = new Matrix4.identity();
+    transform.translate(position.value.x, position.value.y);
+    return new Transform(transform: transform, child: child);
+  }
+}
+
+class ColorAnimation extends AnimatedComponent {
+  ColorAnimation({
+    String key,
+    this.child,
+    this.performance,
+    this.color
+  }) : super(key: key);
+
+  Widget child;
+  AnimatedColor color;
+  AnimationPerformance performance;
+
+  void initState() {
+    watch(performance);
+  }
+
+  void syncFields(ColorAnimation source) {
+    child = source.child;
+    color = source.color;
+    assert(performance == source.performance);
+  }
+
+  Widget build() {
+    return new Container(
+      decoration: new BoxDecoration(backgroundColor: color.value),

[Matt Perry, 2015/07/15 00:20:18]

One of the problems I ran into with animating Material was animating 2 pieces of
a Container. A shadowed Container within a bordered Container is different than
a single Container with shadow+border. Are you planning on adding a
ContainerAnimation?

[abarth-chromium, 2015/07/15 00:23:53]

Yeah, IMHO ColorAnimation should grow to be ContainerAnimation.

+      child: child
+    );
+  }
+}