Add support for lazy transformers.

pkg/barback/lib/src/transformer.dart

 // Copyright (c) 2013, 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.transformer;
 
 import 'dart:async';
 
 import 'asset.dart';
 import 'transform.dart';
 
 /// A [Transformer] represents a processor that takes in one or more input
 /// assets and uses them to generate one or more output assets.
 ///
 /// Dart2js, a SASS->CSS processor, a CSS spriter, and a tool to concatenate
 /// files are all examples of transformers. To define your own transformation
 /// step, extend (or implement) this class.
+///
+/// If possible, transformers should implement [DryRunTransformer] as well to
+/// help barback optimize the package graph.
 abstract class Transformer {
   /// Override this to return a space-separated list of file extensions
   /// (with leading `.`) that are allowed for the primary inputs to this
   /// transformer.
   ///
   /// If you don't override [isPrimary] yourself, it defaults to allowing any
   /// asset whose extension matches one of the ones returned by this. If you
   /// don't override [isPrimary] *or* this, it allows all files.
   String get allowedExtensions => null;
 
@@ skipping 32 matching lines @@
   /// generated assets back to the system. Either can be called multiple times,
   /// in any order.
   ///
   /// In other words, a Transformer's job is to find all inputs for a
   /// transform, starting at the primary input, then generate all output assets
   /// and yield them back to the transform.
   Future apply(Transform transform);
 
   String toString() => runtimeType.toString().replaceAll("Transformer", "");
 }