Roll to pickup pool changes

packages/barback/test/transformer/mock.dart

Index: packages/barback/test/transformer/mock.dart
diff --git a/packages/barback/test/transformer/mock.dart b/packages/barback/test/transformer/mock.dart
index 09c3628efaaeeb97ecf0c15a3cf643f4d15b82df..b6120e250c97fc7fb42777556a75a40a7a1d9a01 100644
--- a/packages/barback/test/transformer/mock.dart
+++ b/packages/barback/test/transformer/mock.dart
@@ -49,7 +49,7 @@ abstract class MockTransformer extends Transformer {
   ///
   /// Once this transformer finishes running, this is reset to a new completer,
   /// so it can be used multiple times.
-  var _started = new Completer();
+  var _started = new Completer<Null>();
 
   /// `true` if any transforms are currently running.
   ///