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/lazy_bad.dart b/pkg/barback/test/transformer/lazy_bad.dart
new file mode 100644
index 0000000000000000000000000000000000000000..4bd07fd9a05bd59d0f274c70b819b13dbf3d9157
--- /dev/null
+++ b/pkg/barback/test/transformer/lazy_bad.dart
@@ -0,0 +1,40 @@
+// 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.test.transformer.lazy_bad;
+
+import 'dart:async';
+
+import 'package:barback/barback.dart';
+import 'package:barback/src/utils.dart';
+
+import 'mock.dart';
+
+/// A lazy transformer that throws an exception after [declareOutputs].
+class LazyBadTransformer extends MockTransformer implements LazyTransformer {
+  /// The error [this] throws.
+  static const ERROR = "I am a bad transformer!";
+
+  /// The asset name that [this] should output.
+  final String output;
+
+  LazyBadTransformer(this.output);
+
+  Future<bool> doIsPrimary(Asset asset) => new Future.value(true);
+
+  Future doApply(Transform transform) {
+    return newFuture(() {
+      var id = new AssetId.parse(output);
+      transform.addOutput(new Asset.fromString(id, output));
+    });
+  }
+
+  Future declareOutputs(DeclaringTransform transform) {
+    return newFuture(() {
+      var id = new AssetId.parse(output);
+      transform.declareOutput(id);
+      throw ERROR;
+    });
+  }
+}