Don't require Transformer methods to return Futures.

pkg/barback/test/transformer/mock.dart

Index: pkg/barback/test/transformer/mock.dart
diff --git a/pkg/barback/test/transformer/mock.dart b/pkg/barback/test/transformer/mock.dart
index 531f9e4dbeb98db4e33da09af412e8acdaab9699..2d13bcd2ce3a282d04b9f0e9ed78773e87725bf2 100644
--- a/pkg/barback/test/transformer/mock.dart
+++ b/pkg/barback/test/transformer/mock.dart
@@ -190,8 +190,13 @@ abstract class MockTransformer extends Transformer {
   }
 
   /// 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);
 }