Refactor fn2.dart, since it breached our 1000-line threshold.

sky/sdk/lib/framework/components2/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 '../fn2.dart';
-import 'dart:async';
-
-typedef void SetterFunction(double value);
-
-class _AnimationEntry {
-  _AnimationEntry(this.value, this.setter);
-  final AnimatedValue value;
-  final SetterFunction setter;
-  StreamSubscription<double> subscription;
-}
-
-abstract class AnimatedComponent extends Component {
-
-  AnimatedComponent({ Object key }) : super(key: key, stateful: true);
-
-  void syncFields(AnimatedComponent source) { }
-
-  List<_AnimationEntry> _animatedFields = new List<_AnimationEntry>();
-
-  animate(AnimatedValue value, SetterFunction setter) {
-    assert(!mounted);
-    setter(value.value);
-    _animatedFields.add(new _AnimationEntry(value, setter));
-  }
-
-  void didMount() {
-    for (_AnimationEntry entry in _animatedFields) {
-      entry.subscription = entry.value.onValueChanged.listen((_) {
-        entry.setter(entry.value.value);
-        scheduleBuild();
-      });
-    }
-    super.didMount();
-  }
-
-  void didUnmount() {
-    for (_AnimationEntry entry in _animatedFields) {
-      assert(entry.subscription != null);
-      entry.subscription.cancel();
-      entry.subscription = null;
-    }
-    super.didUnmount();
-  }
-
-}