Implement an OverscrollCurve for Scrollable

sky/framework/components/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 '../animation/scroll_curve.dart';
 import '../fn.dart';
+import 'dart:math' as math;
 import 'dart:sky' as sky;
 import 'scrollable.dart';
 
 abstract class FixedHeightScrollable extends Scrollable {
   // TODO(rafaelw): This component really shouldn't have an opinion
   // about how it is sized. The owning component should decide whether
   // it's explicitly sized or flexible or whatever...
   static final Style _style = new Style('''
     overflow: hidden;
     position: relative;
@@ skipping 28 matching lines @@
       _itemHeight = itemRect.height;
     });
   }
 
   Node build() {
     var itemNumber = 0;
     var drawCount = 1;
     var transformStyle = '';
 
     if (_height > 0.0) {
-      drawCount = (_height / _itemHeight).round() + 1;
-      double alignmentDelta = -scrollOffset % _itemHeight;
-      if (alignmentDelta != 0.0) {
-        alignmentDelta -= _itemHeight;
+      if (scrollOffset < 0.0) {
+        double visibleHeight = _height + scrollOffset;
+        drawCount = (visibleHeight / _itemHeight).round() + 1;
+        transformStyle =
+          'transform: translateY(${(-scrollOffset).toStringAsFixed(2)}px)';
+      } else {
+        drawCount = (_height / _itemHeight).round() + 1;
+        double alignmentOffset = math.max(0.0, scrollOffset);
+        double alignmentDelta = -scrollOffset % _itemHeight;
+        if (alignmentDelta != 0.0)
+          alignmentDelta -= _itemHeight;
+
+        double drawStart = scrollOffset + alignmentDelta;
+        itemNumber = (drawStart / _itemHeight).floor();
+
+        transformStyle =
+            'transform: translateY(${(alignmentDelta).toStringAsFixed(2)}px)';
       }
-
-      double drawStart = scrollOffset + alignmentDelta;
-      itemNumber = (drawStart / _itemHeight).floor();
-
-      transformStyle =
-          'transform: translateY(${(alignmentDelta).toStringAsFixed(2)}px)';
     }
 
     return new Container(
       styles: [_style],
       children: [
         new Container(
           styles: [_scrollAreaStyle],
           inlineStyle: transformStyle,
           children: buildItems(itemNumber, drawCount)
         )
       ]
     );
   }
 }