Refactor PerformanceView listeners.

Primarily, this rearranges how listeners are handled internally by the
various PerformanceView classes so that there's not so much redundancy.

It also fixes ReversePerformance and ProxyPerformance to not leak.
Previously, they never unhooked their listeners, so they'd leak until
the entire chain of performances was killed. Now, they unhook as soon as
they themselves have no listeners left, which is more idiomatic.

packages/flutter/lib/animation.dart

@@ -8,10 +8,11 @@
 library animation;
 
 export 'src/animation/animated_value.dart';
-export 'src/animation/performance.dart';
 export 'src/animation/clamped_simulation.dart';
 export 'src/animation/curves.dart';
 export 'src/animation/forces.dart';
+export 'src/animation/listener_helpers.dart';
+export 'src/animation/performance.dart';
 export 'src/animation/scroll_behavior.dart';
 export 'src/animation/simulation_stepper.dart';
 export 'src/animation/ticker.dart';

packages/flutter/lib/src/animation/listener_helpers.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 'dart:ui' show VoidCallback;
+
+/// The status of an animation
+enum PerformanceStatus {
+  /// The animation is stopped at the beginning
+  dismissed,
+
+  /// The animation is running from beginning to end
+  forward,
+
+  /// The animation is running backwards, from end to beginning
+  reverse,
+
+  /// The animation is stopped at the end
+  completed,
+}
+
+typedef void PerformanceStatusListener(PerformanceStatus status);
+
+abstract class _ListenerMixin {
+  void didRegisterListener();
+  void didUnregisterListener();
+}
+
+abstract class LazyListenerMixin implements _ListenerMixin {
+  int _listenerCounter = 0;
+  void didRegisterListener() {
+    assert(_listenerCounter >= 0);
+    if (_listenerCounter == 0)
+      didStartListening();
+    _listenerCounter += 1;
+  }
+  void didUnregisterListener() {
+    assert(_listenerCounter >= 1);
+    _listenerCounter -= 1;
+    if (_listenerCounter == 0)
+      didStopListening();
+  }
+  void didStartListening();
+  void didStopListening();
+  bool get isListening => _listenerCounter > 0;
+}
+
+abstract class EagerListenerMixin implements _ListenerMixin {
+  void didRegisterListener() { }
+  void didUnregisterListener() { }
+
+  /// Release any resources used by this object.
+  void dispose();
+}
+
+abstract class LocalPerformanceListenersMixin extends _ListenerMixin {
+  final List<VoidCallback> _listeners = <VoidCallback>[];
+  void addListener(VoidCallback listener) {
+    didRegisterListener();
+    _listeners.add(listener);
+  }
+  void removeListener(VoidCallback listener) {
+    _listeners.remove(listener);
+    didUnregisterListener();
+  }
+  void notifyListeners() {
+    List<VoidCallback> localListeners = new List<VoidCallback>.from(_listeners);
+    for (VoidCallback listener in localListeners)
+      listener();
+  }
+}
+
+abstract class LocalPerformanceStatusListenersMixin extends _ListenerMixin {
+  final List<PerformanceStatusListener> _statusListeners = <PerformanceStatusListener>[];
+  void addStatusListener(PerformanceStatusListener listener) {
+    didRegisterListener();
+    _statusListeners.add(listener);
+  }
+  void removeStatusListener(PerformanceStatusListener listener) {
+    _statusListeners.remove(listener);
+    didUnregisterListener();
+  }
+  void notifyStatusListeners(PerformanceStatus status) {
+    List<PerformanceStatusListener> localListeners = new List<PerformanceStatusListener>.from(_statusListeners);
+    for (PerformanceStatusListener listener in localListeners)
+      listener(status);
+  }
+}

packages/unit/test/widget/page_forward_transitions_test.dart

@@ -13,7 +13,9 @@ class TestTransition extends TransitionComponent {
     this.childFirstHalf,
     this.childSecondHalf,
     PerformanceView performance
-  }) : super(key: key, performance: performance);
+  }) : super(key: key, performance: performance) {
+    assert(performance != null);
+  }
 
   final Widget childFirstHalf;
   final Widget childSecondHalf;