Use package:sky imports consistently

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 'timeline.dart';
-import 'curves.dart';
+import 'package:sky/animation/timeline.dart';
+import 'package:sky/animation/curves.dart';
 
 abstract class AnimatedVariable {
   void setFraction(double t);
 }
 
 class AnimatedType<T extends dynamic> extends AnimatedVariable {
   AnimatedType(this.begin, {this.end, this.curve: linear}) {
     value = begin;
   }
 
@@ skipping 94 matching lines @@
     timeline.stop();
     if (remainingDistance != 0.0)
       timeline.animateTo(target, duration: remainingDistance * duration.inMilliseconds);
   }
 
   void _tick(double t) {
     variable.setFraction(t);
     _notifyListeners();
   }
 }