Stop AnimatedContainer from animating every build

It turns out that an AnimatedContainer with a BoxDecoration would
start animating every single time it was built, whether or not the
container's decoration had changed.

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

@@ -30,6 +30,8 @@ class Scheduler {
   Map<int, SchedulerCallback> _transientCallbacks = new LinkedHashMap<int, SchedulerCallback>();
   final Set<int> _removedIds = new Set<int>();
 
+  int get transientCallbackCount => _transientCallbacks.length;
+
   /// Called by the engine to produce a new frame.
   ///
   /// This function first calls all the callbacks registered by

packages/flutter/lib/src/widgets/animated_container.dart

@@ -66,6 +66,7 @@ class AnimatedContainer extends StatefulComponent {
   }
 
   final Widget child;
+
   final BoxConstraints constraints;
   final BoxDecoration decoration;
   final BoxDecoration foregroundDecoration;
@@ -95,7 +96,7 @@ class _AnimatedContainerState extends State<AnimatedContainer> {
 
   void initState() {
     super.initState();
-    _performance = new Performance(duration: config.duration)
+    _performance = new Performance(duration: config.duration, debugLabel: '${config.toStringShort()}')
       ..timing = new AnimationTiming(curve: config.curve)
       ..addListener(_updateAllVariables);
     _configAllVariables();

packages/flutter/lib/src/widgets/framework.dart

@@ -202,8 +202,12 @@ abstract class Widget {
   /// Inflates this configuration to a concrete instance.
   Element createElement();
 
+  String toStringShort() {
+    return key == null ? '$runtimeType' : '$runtimeType-$key';
+  }
+
   String toString() {
-    final String name = key == null ? '$runtimeType' : '$runtimeType-$key';
+    final String name = toStringShort();
     final List<String> data = <String>[];
     debugFillDescription(data);
     if (data.isEmpty)

packages/unit/test/widget/animated_container_test.dart

+import 'package:flutter/animation.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
 import 'package:test/test.dart';
@@ -45,4 +46,51 @@ void main() {
 
     });
   });
+
+  test('AnimatedContainer overanimate test', () {
+    testWidgets((WidgetTester tester) {
+      tester.pumpWidget(
+        new AnimatedContainer(
+          duration: const Duration(milliseconds: 200),
+          decoration: new BoxDecoration(
+            backgroundColor: new Color(0xFF00FF00)
+          )
+        )
+      );
+      expect(scheduler.transientCallbackCount, 0);
+      tester.pump(new Duration(seconds: 1));
+      expect(scheduler.transientCallbackCount, 0);
+      tester.pumpWidget(
+        new AnimatedContainer(
+          duration: const Duration(milliseconds: 200),
+          decoration: new BoxDecoration(
+            backgroundColor: new Color(0xFF00FF00)
+          )
+        )
+      );
+      expect(scheduler.transientCallbackCount, 0);
+      tester.pump(new Duration(seconds: 1));
+      expect(scheduler.transientCallbackCount, 0);
+      tester.pumpWidget(
+        new AnimatedContainer(
+          duration: const Duration(milliseconds: 200),
+          decoration: new BoxDecoration(
+            backgroundColor: new Color(0xFF0000FF)
+          )
+        )
+      );
+      expect(scheduler.transientCallbackCount, 1); // this is the only time an animation should have started!
+      tester.pump(new Duration(seconds: 1));
+      expect(scheduler.transientCallbackCount, 0);
+      tester.pumpWidget(
+        new AnimatedContainer(
+          duration: const Duration(milliseconds: 200),
+          decoration: new BoxDecoration(
+            backgroundColor: new Color(0xFF0000FF)
+          )
+        )
+      );
+      expect(scheduler.transientCallbackCount, 0);
+    });
+  });
 }