Use package:sky imports consistently

sky/sdk/lib/widgets/popup_menu.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 'dart:math' as math;
 import 'dart:sky' as sky;
 
-import '../animation/animation_performance.dart';
-import '../painting/box_painter.dart';
-import '../theme/colors.dart';
-import '../theme/shadows.dart';
-import 'animated_component.dart';
-import 'basic.dart';
-import 'popup_menu_item.dart';
-import 'scrollable_viewport.dart';
+import 'package:sky/animation/animation_performance.dart';
+import 'package:sky/painting/box_painter.dart';
+import 'package:sky/theme/colors.dart';
+import 'package:sky/theme/shadows.dart';
+import 'package:sky/widgets/animated_component.dart';
+import 'package:sky/widgets/basic.dart';
+import 'package:sky/widgets/popup_menu_item.dart';
+import 'package:sky/widgets/scrollable_viewport.dart';
 
 const Duration _kMenuOpenDuration = const Duration(milliseconds: 300);
 const Duration _kMenuCloseDuration = const Duration(milliseconds: 200);
 const Duration _kMenuCloseDelay = const Duration(milliseconds: 100);
 const double _kMenuWidthStep = 56.0;
 const double _kMenuMargin = 16.0; // 24.0 on tablet
 const double _kMenuMinWidth = 2.0 * _kMenuWidthStep;
 const double _kMenuMaxWidth = 5.0 * _kMenuWidthStep;
 const double _kMenuHorizontalPadding = 16.0;
 const double _kMenuVerticalPadding = 8.0;
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                 )
               )
             )
           )
         )
       )
     );
   }
 
 }