Don't require Transformer methods to return Futures.

pkg/barback/lib/src/declaring_transformer.dart

 // Copyright (c) 2014, 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.declaring_transformer;
 
 import 'dart:async';
 
 import 'declaring_transform.dart';
 
 /// An interface for [Transformer]s that can cheaply figure out which assets
 /// they'll emit without doing the work of actually creating those assets.
 ///
 /// If a transformer implements this interface, that allows barback to perform
 /// optimizations to make the asset graph work more smoothly.
 abstract class DeclaringTransformer {
   /// Declare which assets would be emitted for the primary input id specified
   /// by [transform].
   ///
   /// This works a little like [Transformer.apply], with two main differences.
   /// 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);
 }