Add a TabNavigator to the Stocks demo

sky/sdk/lib/widgets/fixed_height_scrollable.dart

Index: sky/sdk/lib/widgets/fixed_height_scrollable.dart
diff --git a/sky/sdk/lib/widgets/fixed_height_scrollable.dart b/sky/sdk/lib/widgets/fixed_height_scrollable.dart
index 01ac4f9ac362941f2f4aa4269ff6ad3cbfb0d629..1a0997cf7f1ce1e7f1971e643ce2fda49c1b65f9 100644
--- a/sky/sdk/lib/widgets/fixed_height_scrollable.dart
+++ b/sky/sdk/lib/widgets/fixed_height_scrollable.dart
@@ -49,6 +49,14 @@ abstract class FixedHeightScrollable extends Scrollable {
     });
   }
 
+  bool scrollTo(double newScrollOffset) {
+    if (_height != null && _height > 0.0) {
+      double maxScrollOffset = math.max(0.0, itemCount * itemHeight - _height);
+      newScrollOffset = math.min(newScrollOffset, maxScrollOffset);
+    }
+    return super.scrollTo(newScrollOffset);
+  }
+
   Widget buildContent() {
     var itemShowIndex = 0;
     var itemShowCount = 0;