Fix analyzer warning

sky/sdk/example/stocks/lib/stock_home.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import 'package:sky/editing/input.dart';
 import 'package:sky/animation/animation_performance.dart';
 import 'package:sky/widgets/animated_component.dart';
 import 'package:sky/widgets/animated_container.dart';
 import 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/drawer.dart';
@@ skipping 254 matching lines @@
         content: new Text("Stock purchased!"),
         actions: [new SnackBarAction(label: "UNDO", onPressed: _handleUndo)]
       ));
   }
 
   void _handleStockPurchased() {
     setState(() {
       _snackbarTransform = new AnimatedContainer()
         ..position = new AnimatedType<Point>(const Point(0.0, 45.0), end: Point.origin);
       var performance = _snackbarTransform.createPerformance(
-          _snackbarTransform.position, duration: _kSnackbarSlideDuration);
+          [_snackbarTransform.position], duration: _kSnackbarSlideDuration);
       watchPerformance(performance);
       performance.play();
     });
   }
 
   Widget buildFloatingActionButton() {
     var widget = new FloatingActionButton(
       child: new Icon(type: 'content/add_white', size: 24),
       onPressed: _handleStockPurchased
     );
@@ skipping 21 matching lines @@
         body: buildTabNavigator(),
         snackBar: buildSnackBar(),
         floatingActionButton: buildFloatingActionButton(),
         drawer: _drawerShowing ? buildDrawer() : null
       ),
     ];
     addMenuToOverlays(overlays);
     return new Stack(overlays);
   }
 }