Use package:sky imports consistently

sky/sdk/lib/animation/timeline.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 'dart:async';
 
-import '../base/scheduler.dart' as scheduler;
+import 'package:sky/base/scheduler.dart' as scheduler;
 
 class Ticker {
   Ticker(Function onTick) : _onTick = onTick;
 
   final Function _onTick;
 
   Completer _completer;
   int _animationId;
 
   Future start() {
@@ skipping 107 matching lines @@
     final double t = ((timeStamp - _startTime) / _duration).clamp(0.0, 1.0);
     final bool isLastTick = t >= 1.0;
 
     _value = isLastTick ? _end : _begin + (_end - _begin) * t;
     _onTick(_value);
 
     if (isLastTick)
       stop();
   }
 }