Transition barback's infrastructure to an aggregate-based model.

pkg/barback/lib/src/transformer/aggregate_transformer.dart

Index: pkg/barback/lib/src/transformer/aggregate_transformer.dart
diff --git a/pkg/barback/lib/src/transformer/aggregate_transformer.dart b/pkg/barback/lib/src/transformer/aggregate_transformer.dart
index 959b3e75a105331b06982d154b0e83070e7b1da5..bee7bef9fb84e8a68b0ad7b570798cc2436d915c 100644
--- a/pkg/barback/lib/src/transformer/aggregate_transformer.dart
+++ b/pkg/barback/lib/src/transformer/aggregate_transformer.dart
@@ -32,15 +32,16 @@ abstract class AggregateTransformer {
   /// All assets for which [classifyPrimary] returns the same key are passed
   /// together to the same [apply] call.
   ///
-  /// Any string can be used to classify an asset. If possible, though, this
-  /// should return a path-like string to aid in logging. If [classifyPrimary]
-  /// needs to do asynchronous work, it can also return a [Future] that
-  /// completes to the key.
+  /// This may return [Future<String>] or, if it's entirely synchronous,
+  /// [String]. Any string can be used to classify an asset. If possible,
+  /// though, this should return a path-like string to aid in logging. If
+  /// [classifyPrimary] needs to do asynchronous work, it can also return a
+  /// [Future] that completes to the key.

[Bob Nystrom, 2014/05/08 20:30:48]

This last sentence seems redundant with the first. Maybe just reword the first
sentence to clarify the async case?

[nweiz, 2014/05/08 21:12:36]

Done.

   ///
   /// A return value of `null` indicates that the transformer is not interested
   /// in an asset. Assets with a key of `null` will not be passed to any [apply]
   /// call; this is equivalent to [Transformer.isPrimary] returning `false`.
-  String classifyPrimary(AssetId id);
+  classifyPrimary(AssetId id);
 
   /// Runs this transformer on a group of primary inputs specified by
   /// [transform].