Implement an OverscrollCurve for Scrollable

When using OverscrollCurve, we continue to scroll beyond the top of the
scrollable area but the scroll delta is reduced by 2x. A future CL will add an
animation at gesturescrollend to relax back to scroll position 0.0.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1002953003

examples/stocks-fn/stocklist.dart

@@ -8,7 +8,7 @@ class Stocklist extends FixedHeightScrollable {
     Object key,
     this.stocks,
     this.query
-  }) : super(key: key, scrollCurve: new BoundedScrollCurve(minOffset: 0.0));
+  }) : super(key: key, scrollCurve: new OverscrollCurve());
 
   List<Node> buildItems(int start, int count) {
     return stocks