Move VariableHeightScrollable to sdk/lib/widgets

sky/sdk/lib/animation/scroll_behavior.dart

Index: sky/sdk/lib/animation/scroll_behavior.dart
diff --git a/sky/sdk/lib/animation/scroll_behavior.dart b/sky/sdk/lib/animation/scroll_behavior.dart
index b07ee8f7c6d182101d52c90d2d29ee8d2259279e..96ab1653c8d1bdc399bcc1bedffe16b98e5857e1 100644
--- a/sky/sdk/lib/animation/scroll_behavior.dart
+++ b/sky/sdk/lib/animation/scroll_behavior.dart
@@ -60,9 +60,11 @@ class OverscrollBehavior extends ScrollBehavior {
 
   double get maxScrollOffset => math.max(0.0, _contentsHeight - _containerHeight);
 
+  bool get contentsHeightIsKnown => _contentsHeight != null;
+
   Simulation release(Particle particle) {
     System system;
-    if ((particle.position >= 0.0) && (particle.position < maxScrollOffset)) {
+    if (contentsHeightIsKnown && particle.position >= 0.0 && particle.position < maxScrollOffset) {
       if (particle.velocity == 0.0)
         return null;
       System slowdownSystem = new ParticleInBoxWithFriction(
@@ -77,8 +79,18 @@ class OverscrollBehavior extends ScrollBehavior {
           ));
         }));
       system = new Multisystem(particle: particle, system: slowdownSystem);
-    } else {
+    } else if (contentsHeightIsKnown || particle.position < 0) {
       system = getBounceBackSystem(particle);
+    } else {
+      double maxParticlePosition = particle.position + containerHeight;
+      if (particle.velocity == 0.0)
+        return null;
+      System slowdownSystem = new ParticleInBoxWithFriction(
+        particle: particle,
+        friction: _kScrollFriction,
+        box: new GeofenceBox(min: 0.0, max: maxParticlePosition, onEscape: (){ })
+      );
+      system = slowdownSystem;
     }
     return new Simulation(system, terminationCondition: () => particle.position == 0.0);
   }
@@ -90,6 +102,8 @@ class OverscrollBehavior extends ScrollBehavior {
         box: new ClosedBox(max: 0.0),
         theta: _kBounceSlopeAngle,
         targetPosition: 0.0);
+
+    assert(contentsHeightIsKnown);
     return new ParticleClimbingRamp(
       particle: particle,
       box: new ClosedBox(min: maxScrollOffset),
@@ -102,12 +116,12 @@ class OverscrollBehavior extends ScrollBehavior {
     // If we're overscrolling, we want move the scroll offset 2x
     // slower than we would otherwise. Therefore, we "rewind" the
     // newScrollOffset by half the amount that we moved it above.
-    // Notice that we clap the "old" value to 0.0 so that we only
+    // Notice that we clamp the "old" value to 0.0 so that we only
     // reduce the portion of scrollDelta that's applied beyond 0.0. We
     // do similar things for overscroll in the other direction.
     if (newScrollOffset < 0.0) {
       newScrollOffset -= (newScrollOffset - math.min(0.0, scrollOffset)) / 2.0;
-    } else if (newScrollOffset > maxScrollOffset) {
+    } else if (contentsHeightIsKnown && newScrollOffset > maxScrollOffset) {
       newScrollOffset -= (newScrollOffset - math.max(maxScrollOffset, scrollOffset)) / 2.0;
     }
     return newScrollOffset;