Remove calls to [new Future] work around issue 11911.

pkg/barback/test/utils.dart

Index: pkg/barback/test/utils.dart
diff --git a/pkg/barback/test/utils.dart b/pkg/barback/test/utils.dart
index 1dda69732701e3f061a98731eec0cbecf30098bb..fcda96266b3e226e1f6c97f9b8d42f64fb9f8072 100644
--- a/pkg/barback/test/utils.dart
+++ b/pkg/barback/test/utils.dart
@@ -516,7 +516,7 @@ class BadTransformer extends Transformer {
 
   Future<bool> isPrimary(Asset asset) => new Future.value(true);
   Future apply(Transform transform) {
-    return new Future(() {
+    return newFuture(() {
       // Create the outputs first.
       for (var output in outputs) {
         var id = new AssetId.parse(output);
@@ -538,7 +538,7 @@ class CreateAssetTransformer extends Transformer {
   Future<bool> isPrimary(Asset asset) => new Future.value(true);
 
   Future apply(Transform transform) {
-    return new Future(() {
+    return newFuture(() {
       transform.addOutput(new MockAsset(new AssetId.parse(output), output));
     });
   }