Add widgets_next.dart to help folks transition to fn3

Also, fix analyzer warning in fn2.

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

@@ -125,7 +125,7 @@ abstract class Scrollable extends StatefulComponent {
 
   void _startToEndAnimation({ double velocity: 0.0 }) {
     _stopAnimations();
-    Simulation simulation = scrollBehavior.release(scrollOffset, velocity);
+    Simulation simulation = scrollBehavior.createFlingScrollSimulation(scrollOffset, velocity);
     if (simulation != null)
       _toEndAnimation.start(simulation);
   }

packages/flutter/lib/widgets_next.dart

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+/// The Flutter widget framework.
+library widgets;
+
+export 'src/fn3.dart';