Convert Dismissable over to using animated subcomponents

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';
 
@@ skipping 19 matching lines @@
     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 OpacityAnimation extends AnimatedComponent {
+  OpacityAnimation({
+    String key,
+    this.child,
+    this.performance,
+    this.opacity
+  }) : super(key: key);
+
+  Widget child;
+  AnimatedType<double> opacity;
+  AnimationPerformance performance;
+
+  void initState() {
+    watch(performance);

[Matt Perry, 2015/07/15 00:27:26]

maybe have all of these inherit from a common base class which owns the
performance and does the watch()

+  }
+
+  void syncFields(OpacityAnimation source) {
+    child = source.child;
+    opacity = source.opacity;
+    assert(performance == source.performance);
+  }
+
+  Widget build() {
+    return new Opacity(opacity: opacity.value, 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),
       child: child
     );
   }
 }