diff --git a/packages/flutter/lib/src/animation/animations.dart b/packages/flutter/lib/src/animation/animations.dart
index 62490a662c..03d30f530a 100644
--- a/packages/flutter/lib/src/animation/animations.dart
+++ b/packages/flutter/lib/src/animation/animations.dart
@@ -281,13 +281,13 @@ class ReverseAnimation extends Animation<double>
   double get value => 1.0 - parent.value;
 
   AnimationStatus _reverseStatus(AnimationStatus status) {
+    assert(status != null);
     switch (status) {
       case AnimationStatus.forward: return AnimationStatus.reverse;
       case AnimationStatus.reverse: return AnimationStatus.forward;
       case AnimationStatus.completed: return AnimationStatus.dismissed;
       case AnimationStatus.dismissed: return AnimationStatus.completed;
     }
-    assert(status != null);
     return null;
   }
 
@@ -399,7 +399,7 @@ class CurvedAnimation extends Animation<double> with AnimationWithParentMixin<do
     if (reverseCurve == null)
       return '$parent\u27A9$curve';
     if (_useForwardCurve)
-     return '$parent\u27A9$curve\u2092\u2099/$reverseCurve';
+      return '$parent\u27A9$curve\u2092\u2099/$reverseCurve';
     return '$parent\u27A9$curve/$reverseCurve\u2092\u2099';
   }
 }
@@ -438,8 +438,7 @@ class TrainHoppingAnimation extends Animation<double>
     }
     _currentTrain.addStatusListener(_statusChangeHandler);
     _currentTrain.addListener(_valueChangeHandler);
-    if (_nextTrain != null)
-      _nextTrain.addListener(_valueChangeHandler);
+    _nextTrain?.addListener(_valueChangeHandler);
     assert(_mode != null);
   }
 
@@ -479,14 +478,13 @@ class TrainHoppingAnimation extends Animation<double>
           break;
       }
       if (hop) {
-        _currentTrain.removeStatusListener(_statusChangeHandler);
-        _currentTrain.removeListener(_valueChangeHandler);
+        _currentTrain
+          ..removeStatusListener(_statusChangeHandler)
+          ..removeListener(_valueChangeHandler);
         _currentTrain = _nextTrain;
-        // TODO(hixie): This should be setting a status listener on the next
-        // train, not a value listener, and it should pass in _statusChangeHandler,
-        // not _valueChangeHandler
-        _nextTrain.addListener(_valueChangeHandler);
-        _statusChangeHandler(_nextTrain.status);
+        _nextTrain = null;
+        _currentTrain.addStatusListener(_statusChangeHandler);
+        _statusChangeHandler(_currentTrain.status);
       }
     }
     double newValue = value;
@@ -510,10 +508,8 @@ class TrainHoppingAnimation extends Animation<double>
     _currentTrain.removeStatusListener(_statusChangeHandler);
     _currentTrain.removeListener(_valueChangeHandler);
     _currentTrain = null;
-    if (_nextTrain != null) {
-      _nextTrain.removeListener(_valueChangeHandler);
-      _nextTrain = null;
-    }
+    _nextTrain?.removeListener(_valueChangeHandler);
+    _nextTrain = null;
   }
 
   @override
diff --git a/packages/flutter/test/animation/animations_test.dart b/packages/flutter/test/animation/animations_test.dart
index 55a414e429..6217fd8b3a 100644
--- a/packages/flutter/test/animation/animations_test.dart
+++ b/packages/flutter/test/animation/animations_test.dart
@@ -15,9 +15,30 @@ void main() {
   test('toString control test', () {
     expect(kAlwaysCompleteAnimation.toString(), isOneLineDescription);
     expect(kAlwaysDismissedAnimation.toString(), isOneLineDescription);
+    expect(new AlwaysStoppedAnimation<double>(0.5).toString(), isOneLineDescription);
+    CurvedAnimation curvedAnimation = new CurvedAnimation(
+      parent: kAlwaysDismissedAnimation,
+      curve: Curves.ease
+    );
+    expect(curvedAnimation.toString(), isOneLineDescription);
+    curvedAnimation.reverseCurve = Curves.elasticOut;
+    expect(curvedAnimation.toString(), isOneLineDescription);
+    AnimationController controller = new AnimationController(
+      duration: const Duration(milliseconds: 500)
+    );
+    controller
+      ..value = 0.5
+      ..reverse();
+    curvedAnimation = new CurvedAnimation(
+      parent: controller,
+      curve: Curves.ease,
+      reverseCurve: Curves.elasticOut
+    );
+    expect(curvedAnimation.toString(), isOneLineDescription);
+    controller.stop();
   });
 
-  test('toString control test', () {
+  test('ProxyAnimation.toString control test', () {
     ProxyAnimation animation = new ProxyAnimation();
     expect(animation.value, 0.0);
     expect(animation.status, AnimationStatus.dismissed);
@@ -25,4 +46,61 @@ void main() {
     animation.parent = kAlwaysDismissedAnimation;
     expect(animation.toString(), isOneLineDescription);
   });
+
+  test('ProxyAnimation set parent generates value changed', () {
+    AnimationController controller = new AnimationController();
+    controller.value = 0.5;
+    bool didReceiveCallback = false;
+    ProxyAnimation animation = new ProxyAnimation()
+      ..addListener(() {
+        didReceiveCallback = true;
+      });
+    expect(didReceiveCallback, isFalse);
+    animation.parent = controller;
+    expect(didReceiveCallback, isTrue);
+    didReceiveCallback = false;
+    expect(didReceiveCallback, isFalse);
+    controller.value = 0.6;
+    expect(didReceiveCallback, isTrue);
+  });
+
+  test('ReverseAnimation calls listeners', () {
+    AnimationController controller = new AnimationController();
+    controller.value = 0.5;
+    bool didReceiveCallback = false;
+    void listener() {
+      didReceiveCallback = true;
+    }
+    ReverseAnimation animation = new ReverseAnimation(controller)
+      ..addListener(listener);
+    expect(didReceiveCallback, isFalse);
+    controller.value = 0.6;
+    expect(didReceiveCallback, isTrue);
+    didReceiveCallback = false;
+    animation.removeListener(listener);
+    expect(didReceiveCallback, isFalse);
+    controller.value = 0.7;
+    expect(didReceiveCallback, isFalse);
+    expect(animation.toString(), isOneLineDescription);
+  });
+
+  test('TrainHoppingAnimation', () {
+    AnimationController currentTrain = new AnimationController();
+    AnimationController nextTrain = new AnimationController();
+    currentTrain.value = 0.5;
+    nextTrain.value = 0.75;
+    bool didSwitchTrains = false;
+    TrainHoppingAnimation animation = new TrainHoppingAnimation(
+      currentTrain, nextTrain, onSwitchedTrain: () {
+        didSwitchTrains = true;
+      });
+    expect(didSwitchTrains, isFalse);
+    expect(animation.value, 0.5);
+    expect(animation.toString(), isOneLineDescription);
+    nextTrain.value = 0.25;
+    expect(didSwitchTrains, isTrue);
+    expect(animation.value, 0.25);
+    expect(animation.toString(), isOneLineDescription);
+    expect(animation.toString(), contains('no next'));
+  });
 }