[Effen] fix some dart warnings (mostly unused imports)

sky/framework/components/animated_component.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 '../animation/animated_value.dart';
 import '../fn.dart';
 import 'dart:mirrors';
 
-class AnimatedComponent extends Component {
+abstract class AnimatedComponent extends Component {
   AnimatedComponent({ Object key }) : super(key: key, stateful: true);
 
   animateField(AnimatedValue value, Symbol symbol) {
     // TODO(rafaelw): Assert symbol is present on |this|, is private and
     // is over the same parameterized type as the animated value.
     var mirror = reflect(this);
     var subscription;
 
     mirror.setField(symbol, value.value);
 
     onDidMount(() {
       subscription = value.onValueChanged.listen((_) {
         mirror.setField(symbol, value.value);
         scheduleBuild();
       });
     });
 
     onDidUnmount(() {
       if (subscription != null) {
         subscription.cancel();
         subscription = null;
       }
     });
   }
 }