Convert Dismissable over to using animated subcomponents

sky/sdk/lib/widgets/animated.dart

Index: sky/sdk/lib/widgets/animated.dart
diff --git a/sky/sdk/lib/widgets/animated.dart b/sky/sdk/lib/widgets/animated.dart
index 46c5388eed51a05b120d19f4ad527824d1bfeffa..569bd6e76cdfaec5eb1eb57c1f59d982063a6e94 100644
--- a/sky/sdk/lib/widgets/animated.dart
+++ b/sky/sdk/lib/widgets/animated.dart
@@ -37,6 +37,33 @@ class PositionAnimation extends AnimatedComponent {
   }
 }
 
+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,