Fix some dart analyzer warnings.

sky/sdk/lib/widgets/material.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 '../base/lerp.dart';
 import '../animation/animated_value.dart';
 import '../painting/box_painter.dart';
 import '../theme/edges.dart';
 import '../theme/shadows.dart';
 import 'animated_component.dart';
 import 'basic.dart';
 import 'default_text_style.dart';
 import 'theme.dart';
 
 export '../theme/edges.dart' show MaterialEdge;
@@ skipping 51 matching lines @@
         boxShadow: _computeShadow(level.value),
         borderRadius: edges[edge],
         backgroundColor: color,
         shape: edge == MaterialEdge.circle ? Shape.circle : Shape.rectangle
       ),
       child: new DefaultTextStyle(style: Theme.of(this).text.body1, child: child)
     );
   }
 
 }