Add support for lazy transformers.

pkg/barback/test/transformer/lazy_bad.dart

Index: pkg/barback/test/transformer/lazy_bad.dart
diff --git a/pkg/barback/test/transformer/bad.dart b/pkg/barback/test/transformer/lazy_bad.dart
similarity index 52%
copy from pkg/barback/test/transformer/bad.dart
copy to pkg/barback/test/transformer/lazy_bad.dart
index 49046ca5b45aed5e05dda27f5de0121ecd6aac9a..170e0a0f70a05033c5a3bbdcb199064f5f2472ec 100644
--- a/pkg/barback/test/transformer/bad.dart
+++ b/pkg/barback/test/transformer/lazy_bad.dart
@@ -11,28 +11,29 @@ import 'package:barback/src/utils.dart';
 
 import 'mock.dart';
 
-/// A transformer that throws an exception when run, after generating the
-/// given outputs.
-class BadTransformer extends MockTransformer {
-  /// The error it throws.
+/// A lazy transformer that throw an exception after [dryRun].

[Bob Nystrom, 2014/01/30 19:33:44]

throw -> throws

[nweiz, 2014/01/31 03:43:27]

Done.

+class LazyBadTransformer extends MockTransformer implements LazyTransformer {
+  /// The error [this] throws.
   static const ERROR = "I am a bad transformer!";
 
-  /// The list of asset names that it should output.
-  final List<String> outputs;
+  /// The asset name that [this] should output.
+  final String output;
 
-  BadTransformer(this.outputs);
+  LazyBadTransformer(this.output);
 
   Future<bool> doIsPrimary(Asset asset) => new Future.value(true);
 
   Future doApply(Transform transform) {
     return newFuture(() {
-      // Create the outputs first.
-      for (var output in outputs) {
-        var id = new AssetId.parse(output);
-        transform.addOutput(new Asset.fromString(id, output));
-      }
+      var id = new AssetId.parse(output);
+      transform.addOutput(new Asset.fromString(id, output));
+    });
+  }
 
-      // Then fail.
+  Future dryRun(DryRunTransform transform) {
+    return newFuture(() {
+      var id = new AssetId.parse(output);
+      transform.addOutputId(id);
       throw ERROR;
     });
   }