Fix errors in Sky detected by Dart analyzer

sky/framework/components/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 '../fn.dart';
 import '../theme/shadows.dart';
 import 'dart:collection';
-import 'dart:math';
 import 'dart:sky' as sky;
 import 'ink_splash.dart';
 
 class Material extends Component {
-  static const _splashesKey = const Object();
-
   static final Style _splashesStyle = new Style('''
     transform: translateX(0);
     position: absolute;
     top: 0;
     left: 0;
     right: 0;
     bottom: 0'''
   );
 
   static final List<Style> shadowStyle = [
@@ skipping 86 matching lines @@
     }
 
     setState(() {
       _splashes.remove(splash);
       if (_splashes.length == 0) {
         _splashes = null;
       }
     });
   }
 }