Add aggregate transformer types to barback.

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

Index: pkg/barback/lib/src/transformer/lazy_aggregate_transformer.dart
diff --git a/pkg/barback/lib/src/transformer/lazy_transformer.dart b/pkg/barback/lib/src/transformer/lazy_aggregate_transformer.dart
similarity index 59%
copy from pkg/barback/lib/src/transformer/lazy_transformer.dart
copy to pkg/barback/lib/src/transformer/lazy_aggregate_transformer.dart
index 657b65b70bfbe581c8edd22d203b47f6de92de68..891fc1ec29fbf361a7aff0f6b9dfb6247bfb3a3c 100644
--- a/pkg/barback/lib/src/transformer/lazy_transformer.dart
+++ b/pkg/barback/lib/src/transformer/lazy_aggregate_transformer.dart
@@ -2,15 +2,15 @@
 // 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.transformer.lazy_transformer;
+library barback.transformer.lazy_aggregate_transformer;
 
-import 'declaring_transformer.dart';
+import 'declaring_aggregate_transformer.dart';
 
-/// An interface for [Transformer]s that indicates that the transformer's
-/// outputs shouldn't be generated until requested.
+/// An interface for [AggregateTransformer]s that indicates that the
+/// transformer's outputs shouldn't be generated until requested.
 ///
 /// The [declareOutputs] method is used to figure out which assets should be
 /// treated as "lazy." Lazy assets will only be forced to be generated if
 /// they're requested by the user or if they're used by a non-declaring
 /// transformer.
-abstract class LazyTransformer extends DeclaringTransformer {}
+abstract class LazyAggregateTransformer extends DeclaringAggregateTransformer {}

[Bob Nystrom, 2014/05/05 23:41:56]

It seems like the Transformer class hierarchy is starting to get combinatorial.
It's six classes now, right?

If we find ourselves needing another flavor of Transformer class, we may want to
reconsider splitting Declaring out to a separate interface and Lazy to a tag
interface or annotation.

[nweiz, 2014/05/06 22:46:40]

That doesn't sound like an important enough breaking change to warrant
supporting an older version of barback in pub in perpetuity.

I don't think it would even provide much benefit. The class hierarchy encodes
things we want: DeclaringTransformer and DeclaringAggregateTransformer add
different methods than one another, and Lazy* interfaces require that their
respective declare* methods are implemented.