Add support for "flinging" the animation timeline with a spring force. So far,

sky/sdk/lib/animation/animated_simulation.dart

Index: sky/sdk/lib/animation/animated_simulation.dart
diff --git a/sky/sdk/lib/animation/animated_simulation.dart b/sky/sdk/lib/animation/animated_simulation.dart
index 3d40f880f37cbea4d6038cc1549768c7363a142f..f2afb896bdad7a96859f367e6f7f051bd5fff9a3 100644
--- a/sky/sdk/lib/animation/animated_simulation.dart
+++ b/sky/sdk/lib/animation/animated_simulation.dart
@@ -5,10 +5,63 @@
 import 'dart:async';
 
 import 'package:newton/newton.dart';
-import 'package:sky/animation/timeline.dart';
+import 'package:sky/base/scheduler.dart' as scheduler;
 
 const double _kSecondsPerMillisecond = 1000.0;
 
+class Ticker {
+  Ticker(Function onTick) : _onTick = onTick;
+
+  final Function _onTick;
+
+  Completer _completer;
+  int _animationId;
+
+  Future start() {
+    assert(!isTicking);
+    _completer = new Completer();
+    _scheduleTick();
+    return _completer.future;
+  }
+
+  void stop() {
+    if (!isTicking)
+      return;
+
+    if (_animationId != null) {
+      scheduler.cancelAnimationFrame(_animationId);
+      _animationId = null;
+    }
+
+    Completer localCompleter = _completer;
+    _completer = null;
+
+    // We take the _completer into a local variable so that !isTicking when we
+    // actually complete the future.
+    assert(!isTicking);
+    localCompleter.complete();
+  }
+
+  bool get isTicking => _completer != null;
+
+  void _tick(double timeStamp) {
+    assert(isTicking);
+    assert(_animationId != null);
+    _animationId = null;
+
+    _onTick(timeStamp);
+
+    if (isTicking)
+      _scheduleTick();
+  }
+
+  void _scheduleTick() {
+    assert(isTicking);
+    assert(_animationId == null);
+    _animationId = scheduler.requestAnimationFrame(_tick);
+  }
+}
+
 class AnimatedSimulation {
 
   AnimatedSimulation(Function onTick) : _onTick = onTick {
@@ -23,6 +76,11 @@ class AnimatedSimulation {
 
   double _value = 0.0;
   double get value => _value;
+  void set value(double newValue) {
+    assert(!_ticker.isTicking);
+    _value = newValue;
+    _onTick(_value);
+  }
 
   Future start(Simulation simulation) {
     assert(simulation != null);
@@ -36,7 +94,6 @@ class AnimatedSimulation {
   void stop() {
     _simulation = null;
     _startTime = null;
-    _value = 0.0;
     _ticker.stop();
   }
 
@@ -50,10 +107,10 @@ class AnimatedSimulation {
     _value = _simulation.x(timeInSeconds);
     final bool isLastTick = _simulation.isDone(timeInSeconds);
 
-    _onTick(_value);
-
     if (isLastTick)
       stop();
+
+    _onTick(_value);
   }
 
 }