add initState, rename animated_container

sky/sdk/lib/widgets/animation_builder.dart

Index: sky/sdk/lib/widgets/animation_builder.dart
diff --git a/sky/sdk/lib/widgets/animated_container.dart b/sky/sdk/lib/widgets/animation_builder.dart
similarity index 86%
copy from sky/sdk/lib/widgets/animated_container.dart
copy to sky/sdk/lib/widgets/animation_builder.dart
index 89a5a0cb1941cf62df6a67c969c720a323990f50..bbb61d6fd648d3c418107c75d1517d925d874e2a 100644
--- a/sky/sdk/lib/widgets/animated_container.dart
+++ b/sky/sdk/lib/widgets/animation_builder.dart
@@ -14,20 +14,20 @@ import 'basic.dart';
 // This class builds a Container object from a collection of optionally-
 // animated properties. Use syncFields to update the Container's properties,
 // which will optionally animate them using an AnimationPerformance.
-class AnimatedContainer {
+class AnimationBuilder {
   AnimatedType<double> opacity;
   AnimatedType<Point> position;
   AnimatedType<double> shadow;
   AnimatedColor backgroundColor;
 
-  // These don't animate, but are used to build the Container anyway.
+  // These don't animate, but are used to build the AnimationBuilder anyway.
   double borderRadius;
   Shape shape;
 
   Map<AnimatedVariable, AnimationPerformance> _variableToPerformance =
       new Map<AnimatedVariable, AnimationPerformance>();
 
-  AnimatedContainer();
+  AnimationBuilder();
 
   AnimationPerformance createPerformance(List<AnimatedType> variables,
                                          {Duration duration}) {
@@ -65,16 +65,17 @@ class AnimatedContainer {
     return current;
   }
 
-  void syncFields(AnimatedContainer source) {
-    _syncField(position, source.position);
-    _syncField(shadow, source.shadow);
-    _syncField(backgroundColor, source.backgroundColor);
-
-    borderRadius = source.borderRadius;
-    shape = source.shape;
+  void updateFields({ AnimatedType<double> shadow,
+                      AnimatedColor backgroundColor,
+                      double borderRadius,
+                      Shape shape }) {
+    _updateField(this.shadow, shadow);
+    _updateField(this.backgroundColor, backgroundColor);
+    this.borderRadius = borderRadius;
+    this.shape = shape;
   }
 
-  void _syncField(AnimatedType variable, AnimatedType sourceVariable) {
+  void _updateField(AnimatedType variable, AnimatedType sourceVariable) {
     if (variable == null)
       return;  // TODO(mpcomplete): Should we handle transition from null?