Use package:sky imports consistently

sky/sdk/lib/widgets/checkbox.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:sky' as sky;
 
 import 'package:sky/widgets/theme.dart';
+import 'package:sky/widgets/basic.dart';
+import 'package:sky/widgets/toggleable.dart';
 
-import 'basic.dart';
-import 'toggleable.dart';
 export 'toggleable.dart' show ValueChanged;
 
 const double _kMidpoint = 0.5;
 const sky.Color _kLightUncheckedColor = const sky.Color(0x8A000000);
 const sky.Color _kDarkUncheckedColor = const sky.Color(0xB2FFFFFF);
 const double _kEdgeSize = 20.0;
 const double _kEdgeRadius = 1.0;
 
 /// A material design checkbox
 ///
@@ skipping 70 matching lines @@
         => new Point(p1.x * (1.0 - t) + p2.x * t, p1.y * (1.0 - t) + p2.y * t);
       sky.Point drawStart = lerp(start, mid, 1.0 - t);
       sky.Point drawEnd = lerp(mid, end, t);
       path.moveTo(drawStart.x, drawStart.y);
       path.lineTo(mid.x, mid.y);
       path.lineTo(drawEnd.x, drawEnd.y);
       canvas.drawPath(path, paint);
     }
   }
 }