Fix analyzer warning

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

Index: sky/sdk/example/stocks/lib/stock_home.dart
diff --git a/sky/sdk/example/stocks/lib/stock_home.dart b/sky/sdk/example/stocks/lib/stock_home.dart
index 0ef7b1e7118c22bc49f31b60c879afd40de6e829..9d581f0ce5e52efc88b53838e0c5f5f89e5bd198 100644
--- a/sky/sdk/example/stocks/lib/stock_home.dart
+++ b/sky/sdk/example/stocks/lib/stock_home.dart
@@ -272,7 +272,7 @@ class StockHome extends AnimatedComponent {
       _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();
     });