Get rid of the Direction parameter for animation Forces.

Just infer it from the sign of the velocity. That's what odeon does.

packages/flutter/lib/animation/animation_performance.dart

@@ -98,8 +98,10 @@ class AnimationPerformance {
   Future forward() => play(Direction.forward);
   Future reverse() => play(Direction.reverse);
   Future resume() {
-    if (attachedForce != null)
-      return fling(_direction, force: attachedForce);
+    if (attachedForce != null) {
+      return fling(velocity: _direction == Direction.forward ? 1.0 : -1.0,
+                   force: attachedForce);
+    }
     return _animateTo(direction == Direction.forward ? 1.0 : 0.0);
   }
 
@@ -107,13 +109,14 @@ class AnimationPerformance {
     timeline.stop();
   }
 
-  // Flings the timeline in the given direction with an optional force
-  // (defaults to a critically damped spring) and initial velocity.
-  Future fling(Direction direction, {double velocity: 0.0, Force force}) {
+  // Flings the timeline with an optional force (defaults to a critically
+  // damped spring) and initial velocity. If velocity is positive, the
+  // animation will complete, otherwise it will dismiss.
+  Future fling({double velocity: 1.0, Force force}) {
     if (force == null)
       force = kDefaultSpringForce;
-    _direction = direction;
-    return timeline.fling(force.release(progress, velocity, _direction));
+    _direction = velocity < 0.0 ? Direction.reverse : Direction.forward;
+    return timeline.fling(force.release(progress, velocity));
   }
 
   final List<Function> _listeners = new List<Function>();

packages/flutter/lib/animation/forces.dart

@@ -3,11 +3,10 @@
 // found in the LICENSE file.
 
 import 'package:newton/newton.dart';
-import 'package:sky/animation/direction.dart';
 
 // Base class for creating Simulations for the animation Timeline.
 abstract class Force {
-  Simulation release(double position, double velocity, Direction direction);
+  Simulation release(double position, double velocity);
 }
 
 class SpringForce extends Force {
@@ -24,8 +23,8 @@ class SpringForce extends Force {
   // respectively.
   final double left, right;
 
-  Simulation release(double position, double velocity, Direction direction) {
-    double target = direction == Direction.reverse ?
+  Simulation release(double position, double velocity) {
+    double target = velocity < 0.0 ?
         this.left - tolerance.distance : this.right + tolerance.distance;
     return new SpringSimulation(spring, position, target, velocity)
       ..tolerance = tolerance;

packages/flutter/lib/widgets/dismissable.dart

@@ -153,7 +153,7 @@ class Dismissable extends StatefulComponent {
     if (_isHorizontalFlingGesture(event)) {
       _dragUnderway = false;
       _dragX = event.velocityX.sign;
-      _fadePerformance.fling(Direction.forward, velocity: event.velocityX.abs() * _kFlingVelocityScale);
+      _fadePerformance.fling(velocity: event.velocityX.abs() * _kFlingVelocityScale);
     }
     return EventDisposition.processed;
   }

packages/flutter/lib/widgets/drawer.dart

@@ -171,9 +171,7 @@ class Drawer extends StatefulComponent {
 
   EventDisposition handleFlingStart(event) {
     if (event.velocityX.abs() >= _kMinFlingVelocity) {
-      _performance.fling(
-          event.velocityX < 0.0 ? Direction.reverse : Direction.forward,
-          velocity: event.velocityX.abs() * _kFlingVelocityScale);
+      _performance.fling(velocity: event.velocityX * _kFlingVelocityScale);
       return EventDisposition.processed;
     }
     return EventDisposition.ignored;