Fix remaining analyzer warnings in Sky

sky/sdk/lib/animation/animation_performance.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import 'animated_value.dart';
 import 'curves.dart';
 
 // TODO(mpcomplete): merge this stuff with AnimatedValue somehow. We shouldn't
 // have 2 different ways to animate values.
 abstract class AnimatedVariable {
   void setFraction(double t);
 }
 
-class AnimatedType<T> extends AnimatedVariable {
+class AnimatedType<T extends dynamic> extends AnimatedVariable {
   T value;
   final T begin, end;
   final Curve curve;
 
   AnimatedType(this.begin, this.end, {this.curve: linear}) {
     value = begin;
   }
 
   void setFraction(double t) {
     // TODO(mpcomplete): Reverse the timeline and curve.
@@ skipping 55 matching lines @@
   // time.
   void fling({double velocity: 1.0}) {
     double target = velocity.sign < 0.0 ? 0.0 : 1.0;
     double distance = (target - timeline.value).abs();
     double duration = distance / velocity.abs();
 
     if (distance > 0.0)
       timeline.animateTo(target, duration, curve: linear);
   }
 }