Factor ScrollCurve out of Scrollable

sky/examples/stocks-fn/stocklist.dart

 part of stocksapp;
 
 class Stocklist extends FixedHeightScrollable {
   String query;
   List<Stock> stocks;
 
   Stocklist({
     Object key,
     this.stocks,
     this.query
-  }) : super(key: key, minOffset: 0.0);
+  }) : super(key: key, scrollCurve: new BoundedScrollCurve(minOffset: 0.0));
 
   List<Node> buildItems(int start, int count) {
     return stocks
       .skip(start)
       .where((stock) => query == null || stock.symbol.contains(
           new RegExp(query, caseSensitive: false)))
       .take(count)
       .map((stock) => new StockRow(stock: stock))
       .toList(growable: false);
   }
 }