Ensure that sky Nodes are in the document when didMount() is fired. Refactor FixedHeightScrollable …

sky/examples/stocks-fn/stocklist.dart

 part of stocksapp;
 
 class Stocklist extends FixedHeightScrollable {
 
   List<Stock> stocks;
 
   Stocklist({
     Object key,
     this.stocks
-  }) : super(key: key, itemHeight: 80.0, height: 800.0, minOffset: 0.0);
+  }) : super(key: key, minOffset: 0.0);
 
   List<Node> renderItems(int start, int count) {
     var items = [];
     for (var i = 0; i < count; i++) {
       items.add(new StockRow(stock: stocks[start + i]));
     }
 
     return items;
   }
 }