AnimationController reset() method (#13044)

* AnimationController reset() method

Just a simple convenience method to fix #13039

* Added `reset()` test

* More test expectations

Per feedback.

* Removed test print

* Improved documentation of reset()

* Add controller.reverse to test

packages/flutter/lib/src/animation/animation_controller.dart

@@ -218,6 +218,12 @@ class AnimationController extends Animation<double>
     notifyListeners();
     _checkStatusChanged();
   }
+  
+  /// Sets the controller's value to [lowerBound], stopping the animation (if
+  /// in progress), and resetting to its beginning point, or dismissed state.
+  void reset() {
+    value = lowerBound;
+  }
 
   /// The rate of change of [value] per second.
   ///

packages/flutter/test/animation/animation_controller_test.dart

@@ -396,6 +396,59 @@ void main() {
     expect(controller.value, 1.0);
   });
 
+  test('resetting animation works at all phases', (){
+    final List<AnimationStatus> statusLog = <AnimationStatus>[];
+    final AnimationController controller = new AnimationController(
+      duration: const Duration(milliseconds: 100),
+      value: 0.0,
+      lowerBound: 0.0,
+      upperBound: 1.0,
+      vsync: const TestVSync(),
+    )..addStatusListener(statusLog.add);
+
+    expect(controller.value, 0.0);
+    expect(controller.status, AnimationStatus.dismissed);
+
+    controller.reset();
+
+    expect(controller.value, 0.0);
+    expect(controller.status, AnimationStatus.dismissed);
+
+    statusLog.clear();
+    controller.forward();
+    tick(const Duration(milliseconds: 0));
+    tick(const Duration(milliseconds: 50));
+    expect(controller.status, AnimationStatus.forward);
+    controller.reset();
+
+    expect(controller.value, 0.0);
+    expect(controller.status, AnimationStatus.dismissed);
+    expect(statusLog, equals(<AnimationStatus>[ AnimationStatus.forward, AnimationStatus.dismissed ]));
+
+    controller.value = 1.0;
+    statusLog.clear();
+    controller.reverse();
+    tick(const Duration(milliseconds: 0));
+    tick(const Duration(milliseconds: 50));
+    expect(controller.status, AnimationStatus.reverse);
+    controller.reset();
+
+    expect(controller.value, 0.0);
+    expect(controller.status, AnimationStatus.dismissed);
+    expect(statusLog, equals(<AnimationStatus>[ AnimationStatus.reverse, AnimationStatus.dismissed ]));
+
+    statusLog.clear();
+    controller.forward();
+    tick(const Duration(milliseconds: 0));
+    tick(const Duration(milliseconds: 150));
+    expect(controller.status, AnimationStatus.completed);
+    controller.reset();
+
+    expect(controller.value, 0.0);
+    expect(controller.status, AnimationStatus.dismissed);
+    expect(statusLog, equals(<AnimationStatus>[ AnimationStatus.forward, AnimationStatus.completed, AnimationStatus.dismissed ]));
+  });
+
   test('setting value directly sets correct status', () {
     final AnimationController controller = new AnimationController(
       value: 0.0,