Refactor the barback tests somewhat.

pkg/barback/test/transformer/bad.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.
+
+import 'dart:async';
+
+import 'package:barback/barback.dart';
+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.
+  static const ERROR = "I am a bad transformer!";
+
+  /// The list of asset names that it should output.
+  final List<String> outputs;
+
+  BadTransformer(this.outputs);
+
+  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));
+      }
+
+      // Then fail.
+      throw ERROR;
+    });
+  }
+}