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

sky/framework/animation/animated_value.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 'curves.dart';
 import 'dart:async';
 import 'generators.dart';
 
 class AnimatedValue {
   StreamController _controller = new StreamController(sync: true);
   AnimationGenerator _animation;
   Completer _completer;
   double _value;
 
@@ skipping 52 matching lines @@
     return _completer.future;
   }
 
   double get remainingTime {
     if (_animation == null)
       return 0.0;
     return _animation.remainingTime;
   }
 
 }