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

pkg/barback/lib/src/asset_cascade.dart

Index: pkg/barback/lib/src/asset_cascade.dart
diff --git a/pkg/barback/lib/src/asset_cascade.dart b/pkg/barback/lib/src/asset_cascade.dart
index 0e8893a23ddd7c4a8eb83284ab1537901052227e..3d6416f5f6c5953d60f4deb5ceaa213de49b7b1a 100644
--- a/pkg/barback/lib/src/asset_cascade.dart
+++ b/pkg/barback/lib/src/asset_cascade.dart
@@ -223,7 +223,7 @@ class AssetCascade {
     // Always pump the event loop. This ensures a bunch of synchronous source
     // changes are processed in a single batch even when the first one starts
     // the build process.
-    return new Future(() {
+    return newFuture(() {
       if (_sourceChanges == null) return null;
 
       // Take the current batch to ensure it doesn't get added to while we're