Add a TabNavigator to the Stocks demo

sky/sdk/lib/widgets/fixed_height_scrollable.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.
 
 import 'dart:math' as math;
 
 import 'package:vector_math/vector_math.dart';
 
 import '../animation/scroll_behavior.dart';
 import 'basic.dart';
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   }
 
   double _height;
   void _handleSizeChanged(Size newSize) {
     setState(() {
       _height = newSize.height;
       scrollBehavior.containerHeight = _height;
     });
   }
 
+  bool scrollTo(double newScrollOffset) {
+    if (_height != null && _height > 0.0) {
+      double maxScrollOffset = math.max(0.0, itemCount * itemHeight - _height);
+      newScrollOffset = math.min(newScrollOffset, maxScrollOffset);
+    }
+    return super.scrollTo(newScrollOffset);
+  }
+
   Widget buildContent() {
     var itemShowIndex = 0;
     var itemShowCount = 0;
 
     Matrix4 transform = new Matrix4.identity();
 
     if (_height != null && _height > 0.0) {
       if (scrollOffset < 0.0) {
         double visibleHeight = _height + scrollOffset;
         itemShowCount = (visibleHeight / itemHeight).round() + 1;
@@ skipping 24 matching lines @@
             child: new Block(items)
           )
         )
       )
     );
   }
 
   List<Widget> buildItems(int start, int count);
 
 }