Move scroll_behavior into widgets

Previously this code was in the animation layer, which didn't make much
sense because scrolling is a widget concern.

packages/flutter/lib/animation.dart

@@ -13,6 +13,5 @@ export 'src/animation/animations.dart';
 export 'src/animation/curves.dart';
 export 'src/animation/forces.dart';
 export 'src/animation/listener_helpers.dart';
-export 'src/animation/scroll_behavior.dart';
 export 'src/animation/ticker.dart';
 export 'src/animation/tween.dart';

packages/flutter/lib/src/material/input.dart

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'package:flutter/animation.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter/widgets.dart';

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

@@ -10,6 +10,7 @@ import 'package:flutter/rendering.dart';
 
 import 'basic.dart';
 import 'framework.dart';
+import 'scroll_behavior.dart';
 import 'scrollable.dart';
 import 'virtual_viewport.dart';
 

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

@@ -18,6 +18,7 @@ import 'homogeneous_viewport.dart';
 import 'mixed_viewport.dart';
 import 'notification_listener.dart';
 import 'page_storage.dart';
+import 'scroll_behavior.dart';
 
 // The gesture velocity properties are pixels/second, config min,max limits are pixels/ms
 const double _kMillisecondsPerSecond = 1000.0;

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

@@ -4,13 +4,13 @@
 
 import 'dart:math' as math;
 
+import 'package:flutter/rendering.dart';
+
 import 'framework.dart';
+import 'scroll_behavior.dart';
 import 'scrollable.dart';
 import 'virtual_viewport.dart';
 
-import 'package:flutter/animation.dart';
-import 'package:flutter/rendering.dart';
-
 /// A vertically scrollable grid.
 ///
 /// Requires that delegate places its children in row-major order.

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

@@ -5,10 +5,10 @@
 import 'dart:math' as math;
 
 import 'framework.dart';
+import 'scroll_behavior.dart';
 import 'scrollable.dart';
 import 'virtual_viewport.dart';
 
-import 'package:flutter/animation.dart';
 import 'package:flutter/rendering.dart';
 
 class ScrollableList extends Scrollable {

packages/flutter/lib/widgets.dart

@@ -35,6 +35,7 @@ export 'src/widgets/routes.dart';
 export 'src/widgets/scrollable.dart';
 export 'src/widgets/scrollable_grid.dart';
 export 'src/widgets/scrollable_list.dart';
+export 'src/widgets/scroll_behavior.dart';
 export 'src/widgets/status_transitions.dart';
 export 'src/widgets/title.dart';
 export 'src/widgets/transitions.dart';

packages/flutter/test/widget/remember_scroll_position_test.dart

@@ -3,7 +3,6 @@
 // found in the LICENSE file.
 
 import 'package:flutter_test/flutter_test.dart';
-import 'package:flutter/animation.dart';
 import 'package:flutter/material.dart';
 import 'package:test/test.dart';