fix back button in stocks app by calling super.didMount()

TBR=abarth

Review URL: https://codereview.chromium.org/1189113004.

examples/stocks2/lib/stock_app.dart

@@ -27,6 +27,7 @@ class StocksApp extends App {
   }
 
   void didMount() {
+    super.didMount();
     new StockDataFetcher((StockData data) {
       setState(() {
         data.appendTo(_stocks);