fn-drawer sometimes doesn't tick closed

sky/examples/fn/widgets/animationgenerator.dart

 part of widgets;
 
 class FrameGenerator {
 
   Function onDone;
   StreamController _controller;
 
   Stream<double> get onTick => _controller.stream;
 
   int _animationId = 0;
@@ skipping 27 matching lines @@
 
   void _tick(double timeStamp) {
     _animationId = 0;
     _controller.add(timeStamp);
     if (!_cancelled) {
       _scheduleTick();
     }
   }
 }
 
-const double _kFrameTime = 1000 / 60;
-
 class AnimationGenerator extends FrameGenerator {
 
   Stream<double> get onTick => _stream;
   final double duration;
   final double begin;
   final double end;
   final Curve curve;
   Stream<double> _stream;
+  bool _done = false;
 
   AnimationGenerator(this.duration, {
     this.begin: 0.0,
     this.end: 1.0,
     this.curve: linear,
     Function onDone
   }):super(onDone: onDone) {
     double startTime = 0.0;
     double targetTime = 0.0;
-    bool done = false;
     _stream = super.onTick.map((timeStamp) {
       if (startTime == 0.0) {
         startTime = timeStamp;
         targetTime = startTime + duration;
       }
+      return math.min((timeStamp - startTime) / duration, 1.0);
+    })
+    .takeWhile(_checkForCompletion)
+    .map((t) =>  begin + (end - begin) * curve.transform(t));
+  }
 
-      // Clamp the final frame to target time so we terminate the series with
-      // 1.0 exactly.
-      if ((timeStamp - targetTime).abs() <= _kFrameTime) {
-        return 1.0;
-      }
-
-      return (timeStamp - startTime) / duration;
-    })
-    .takeWhile((t) => t <= 1.0)
-    .map((t) =>  begin + (end - begin) * curve.transform(t));
+  bool _checkForCompletion(double t) {
+    if (_done)
+      return false;
+    _done = t >= 1;
+    return true;
   }
 }
 
 double _evaluateCubic(double a, double b, double m) {
   // TODO(abarth): Would Math.pow be faster?
   return 3 * a * (1 - m) * (1 - m) * m + 3 * b * (1 - m) * m * m + m * m * m;
 }
 
 const double _kCubicErrorBound = 0.001;
 
@@ skipping 36 matching lines @@
         end = midpoint;
     }
   }
 }
 
 const Linear linear = const Linear();
 const Cubic ease = const Cubic(0.25, 0.1, 0.25, 1.0);
 const Cubic easeIn = const Cubic(0.42, 0.0, 1.0, 1.0);
 const Cubic easeOut = const Cubic(0.0, 0.0, 0.58, 1.0);
 const Cubic easeInOut = const Cubic(0.42, 0.0, 0.58, 1.0);