Add a [Transform.hasInput] convenience method to Barback.

pkg/barback/test/transformer/has_input.dart

Index: pkg/barback/test/transformer/has_input.dart
diff --git a/pkg/barback/test/transformer/has_input.dart b/pkg/barback/test/transformer/has_input.dart
new file mode 100644
index 0000000000000000000000000000000000000000..add301724bad8b65032f7b2832b82b1f2bacd95a
--- /dev/null
+++ b/pkg/barback/test/transformer/has_input.dart
@@ -0,0 +1,34 @@
+// 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.has_input;
+
+import 'dart:async';
+
+import 'package:barback/barback.dart';
+
+import 'mock.dart';
+
+/// Overwrites its primary inputs with descriptions of whether various secondary
+/// inputs exist.
+class HasInputTransformer extends MockTransformer {
+  /// The inputs whose existence will be checked.
+  final List<AssetId> inputs;
+
+  HasInputTransformer(Iterable<String> inputs)
+      : inputs = inputs.map((input) => new AssetId.parse(input)).toList();
+
+  Future<bool> doIsPrimary(Asset asset) => new Future.value(true);
+
+  Future doApply(Transform transform) {
+    return Future.wait(inputs.map((input) {
+      return transform.hasInput(input).then((hasInput) => "$input: $hasInput");
+    })).then((results) {
+      transform.addOutput(new Asset.fromString(
+          transform.primaryInput.id, results.join(', ')));
+    });
+  }
+
+  String toString() => "has inputs $inputs";
+}