Don't require Transformer methods to return Futures.

pkg/barback/test/transformer/mock.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library barback.test.transformer.mock;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 import 'package:barback/src/utils.dart';
@@ skipping 172 matching lines @@
     if (consumePrimary) transform.consumePrimary();
     return newFuture(() => doApply(transform)).then((_) {
       if (_apply != null) return _apply.future;
     }).whenComplete(() {
       _runningTransforms--;
       if (_runningTransforms == 0) _started = new Completer();
     });
   }
 
   /// The wrapped version of [isPrimary] for subclasses to override.
-  Future<bool> doIsPrimary(AssetId id);
+  ///
+  /// This can return a `bool` or a `Future<bool>`.
+  doIsPrimary(AssetId id);
 
   /// The wrapped version of [doApply] for subclasses to override.
-  Future doApply(Transform transform);
+  ///
+  /// This may return a [Future]. If it does, it won't be considered finished
+  /// until that [Future] completes.
+  doApply(Transform transform);
 }