Fix some dart analyzer warnings.

sky/sdk/lib/widgets/drawer.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:math' as math;
 import 'dart:sky' as sky;
 
-import 'package:vector_math/vector_math.dart';
-
-import '../animation/animated_value.dart';
 import '../animation/animation_performance.dart';
 import '../animation/curves.dart';
 import '../theme/colors.dart';
 import '../theme/shadows.dart';
 import 'animated_component.dart';
 import 'basic.dart';
 
 // TODO(eseidel): Draw width should vary based on device size:
 // http://www.google.com/design/spec/layout/structure.html#structure-side-nav
 
@@ skipping 121 matching lines @@
       child: new Stack([ mask, content ]),
       onPointerDown: controller.handlePointerDown,
       onPointerMove: controller.handlePointerMove,
       onPointerUp: controller.handlePointerUp,
       onPointerCancel: controller.handlePointerCancel,
       onGestureFlingStart: controller.handleFlingStart
     );
   }
 
 }