Don't require Transformer methods to return Futures.

pkg/barback/lib/src/declaring_transformer.dart

Index: pkg/barback/lib/src/declaring_transformer.dart
diff --git a/pkg/barback/lib/src/declaring_transformer.dart b/pkg/barback/lib/src/declaring_transformer.dart
index a784493da9cf3268665ff14b545e6a433379da28..5062907d98b001f335c2a3e3afa0c05c3db267c2 100644
--- a/pkg/barback/lib/src/declaring_transformer.dart
+++ b/pkg/barback/lib/src/declaring_transformer.dart
@@ -21,5 +21,8 @@ abstract class DeclaringTransformer {
   /// First, instead of having access to the primary input's contents, it only
   /// has access to its id. Second, instead of emitting [Asset]s, it just emits
   /// [AssetId]s through [transform.addOutputId].
-  Future declareOutputs(DeclaringTransform transform);
+  ///
+  /// This may return a [Future]. If it does, it won't be considered finished
+  /// until that [Future] completes.

[Bob Nystrom, 2014/04/21 22:32:23]

I think we need to word this more strongly. Users need to understand that if the
body of this does asynchronous work, it *should* return a future.

[nweiz, 2014/04/21 22:46:24]

Done.

+  declareOutputs(DeclaringTransform transform);
 }