Move AnimatedContainer to a new file so AnimatedPositioned can live with it.

packages/flutter/lib/widgets.dart

@@ -5,7 +5,6 @@
 /// The Flutter widget framework.
 library widgets;
 
-export 'src/widgets/animated_container.dart';
 export 'src/widgets/basic.dart';
 export 'src/widgets/binding.dart';
 export 'src/widgets/dismissable.dart';
@@ -18,6 +17,7 @@ export 'src/widgets/gesture_detector.dart';
 export 'src/widgets/gridpaper.dart';
 export 'src/widgets/heroes.dart';
 export 'src/widgets/homogeneous_viewport.dart';
+export 'src/widgets/implicit_animations.dart';
 export 'src/widgets/locale_query.dart';
 export 'src/widgets/media_query.dart';
 export 'src/widgets/mimic.dart';