Add a ValueAnimation helper class for AnimationPerfomance.

Used for AnimationPerformance with a single AnimatedValue<T> variable.

examples/widgets/ensure_visible.dart

@@ -38,7 +38,7 @@ class EnsureVisibleApp extends App {
   List<CardModel> cardModels;
   BlockViewportLayoutState layoutState = new BlockViewportLayoutState();
   ScrollListener scrollListener;
-  AnimationPerformance scrollAnimation;
+  ValueAnimation<double> scrollAnimation;
 
   void initState() {
     List<double> cardHeights = <double>[
@@ -51,7 +51,7 @@ class EnsureVisibleApp extends App {
       return new CardModel(i, cardHeights[i], color);
     });
 
-    scrollAnimation = new AnimationPerformance()
+    scrollAnimation = new ValueAnimation<double>()
       ..duration = const Duration(milliseconds: 200)
       ..variable = new AnimatedValue<double>(0.0, curve: ease);
 

packages/flutter/lib/animation/animation_performance.dart

@@ -26,13 +26,17 @@ enum AnimationStatus {
 // manipulating |progress|, or |fling| the timeline causing a physics-based
 // simulation to take over the progression.
 class AnimationPerformance {
-  AnimationPerformance({this.variable, this.duration}) {
+  AnimationPerformance({AnimatedVariable variable, this.duration}) :
+    _variable = variable {
     _timeline = new Timeline(_tick);
   }
 
-  AnimatedVariable variable;
+  AnimatedVariable _variable;
   Duration duration;
 
+  AnimatedVariable get variable => _variable;
+  void set variable(AnimatedVariable v) { _variable = v; }
+
   // Advances from 0 to 1. On each tick, we'll update our variable's values.
   Timeline _timeline;
   Timeline get timeline => _timeline;
@@ -179,3 +183,14 @@ class AnimationPerformance {
     _checkStatusChanged();
   }
 }
+
+// Simple helper class for an animation with a single value.
+class ValueAnimation<T> extends AnimationPerformance {
+  ValueAnimation({AnimatedValue<T> variable, Duration duration}) :
+    super(variable: variable, duration: duration);
+
+  AnimatedValue<T> get variable => _variable as AnimatedValue<T>;
+  void set variable(AnimatedValue<T> v) { _variable = v; }
+
+  T get value => variable.value;
+}

packages/flutter/lib/widgets/scrollable.dart

@@ -7,7 +7,6 @@ import 'dart:sky' as sky;
 
 import 'package:newton/newton.dart';
 import 'package:sky/animation/animated_simulation.dart';
-import 'package:sky/animation/animated_value.dart';
 import 'package:sky/animation/animation_performance.dart';
 import 'package:sky/animation/scroll_behavior.dart';
 import 'package:sky/rendering/box.dart';
@@ -44,7 +43,7 @@ abstract class Scrollable extends StatefulComponent {
   ScrollDirection scrollDirection;
 
   AnimatedSimulation _toEndAnimation; // See _startToEndAnimation()
-  AnimationPerformance _toOffsetAnimation; // Started by scrollTo()
+  ValueAnimation<double> _toOffsetAnimation; // Started by scrollTo()
 
   void initState() {
     _toEndAnimation = new AnimatedSimulation(_tickScrollOffset);
@@ -86,11 +85,11 @@ abstract class Scrollable extends StatefulComponent {
     );
   }
 
-  void _startToOffsetAnimation(double newScrollOffset, AnimationPerformance animation) {
+  void _startToOffsetAnimation(double newScrollOffset, ValueAnimation<double> animation) {
     _stopToEndAnimation();
     _stopToOffsetAnimation();
 
-    (animation.variable as AnimatedValue<double>)
+    animation.variable
       ..begin = scrollOffset
       ..end = newScrollOffset;
 
@@ -102,8 +101,7 @@ abstract class Scrollable extends StatefulComponent {
   }
 
   void _updateToOffsetAnimation() {
-    AnimatedValue<double> offset = _toOffsetAnimation.variable;
-    scrollTo(offset.value);
+    scrollTo(_toOffsetAnimation.value);
   }
 
   void _updateToOffsetAnimationStatus(AnimationStatus status) {
@@ -139,7 +137,7 @@ abstract class Scrollable extends StatefulComponent {
     super.didUnmount();
   }
 
-  bool scrollTo(double newScrollOffset, { AnimationPerformance animation }) {
+  bool scrollTo(double newScrollOffset, { ValueAnimation<double> animation }) {
     if (newScrollOffset == _scrollOffset)
       return false;
 
@@ -233,7 +231,7 @@ Scrollable findScrollableAncestor({ Widget target }) {
   return ancestor;
 }
 
-bool ensureWidgetIsVisible(Widget target, { AnimationPerformance animation }) {
+bool ensureWidgetIsVisible(Widget target, { ValueAnimation<double> animation }) {
   assert(target.mounted);
   assert(target.root is RenderBox);
 

packages/flutter/lib/widgets/tabs.dart

@@ -406,15 +406,15 @@ class TabBar extends Scrollable {
 
   Size _tabBarSize;
   List<double> _tabWidths;
-  AnimationPerformance _indicatorAnimation;
-  AnimationPerformance _scrollAnimation;
+  ValueAnimation<Rect> _indicatorAnimation;
+  ValueAnimation<double> _scrollAnimation;
 
   void initState() {
     super.initState();
-    _indicatorAnimation = new AnimationPerformance()
+    _indicatorAnimation = new ValueAnimation<Rect>()
       ..duration = _kTabBarScroll
       ..variable = new AnimatedRect(null, curve: ease);
-    _scrollAnimation = new AnimationPerformance()
+    _scrollAnimation = new ValueAnimation<double>()
       ..duration = _kTabBarScroll
       ..variable = new AnimatedValue<double>(0.0, curve: ease);
   }
@@ -430,7 +430,7 @@ class TabBar extends Scrollable {
     scrollBehavior.isScrollable = source.isScrollable;
   }
 
-  AnimatedRect get _indicatorRect => _indicatorAnimation.variable as AnimatedRect;
+  AnimatedRect get _indicatorRect => _indicatorAnimation.variable;
 
   void _startIndicatorAnimation(int fromTabIndex, int toTabIndex) {
     _indicatorRect