Use package:sky imports consistently

sky/sdk/lib/widgets/animated_container.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 'package:vector_math/vector_math.dart';
 
-import '../animation/animation_performance.dart';
-import '../animation/curves.dart';
-import '../base/lerp.dart';
-import '../painting/box_painter.dart';
-import '../theme/shadows.dart';
-import 'basic.dart';
+import 'package:sky/animation/animation_performance.dart';
+import 'package:sky/animation/curves.dart';
+import 'package:sky/base/lerp.dart';
+import 'package:sky/painting/box_painter.dart';
+import 'package:sky/theme/shadows.dart';
+import 'package:sky/widgets/basic.dart';
 
 // This class builds a Builder object from a collection of optionally-
 // animated properties. Use syncFields to update the Builder's properties,
 // which will optionally animate them using an AnimationPerformance.
 class AnimationBuilder {
   AnimatedType<double> opacity;
   AnimatedType<Point> position;
   AnimatedType<double> shadow;
   AnimatedColor backgroundColor;
 
@@ skipping 89 matching lines @@
 
   int level1 = level.floor();
   int level2 = level.ceil();
   double t = level - level1.toDouble();
 
   List<BoxShadow> shadow = new List<BoxShadow>();
   for (int i = 0; i < shadows[level1].length; ++i)
     shadow.add(lerpBoxShadow(shadows[level1][i], shadows[level2][i], t));
   return shadow;
 }