fix the hang when clicking the drawer in the stocks app

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/980043005

examples/fn/widgets/drawer.dart

@@ -73,9 +73,11 @@ class DrawerAnimation {
 
   void _animateToPosition(double targetPosition) {
     double distance = (targetPosition - _position).abs();
-    double targetDuration = distance / _kWidth * _kBaseSettleDurationMS;
-    double duration = math.min(targetDuration, _kMaxSettleDurationMS);
-    _animate(duration, _position, targetPosition, _kAnimationCurve);
+    if (distance != 0) {
+      double targetDuration = distance / _kWidth * _kBaseSettleDurationMS;
+      double duration = math.min(targetDuration, _kMaxSettleDurationMS);
+      _animate(duration, _position, targetPosition, _kAnimationCurve);
+    }
   }
 
   void handleFlingStart(event) {

framework/animation/generator.dart

@@ -66,6 +66,7 @@ class AnimationGenerator extends FrameGenerator {
     this.curve: linear,
     Function onDone
   }):super(onDone: onDone) {
+    assert(duration > 0);
     double startTime = 0.0;
     _stream = super.onTick.map((timeStamp) {
       if (startTime == 0.0)