add benchmarks.dart which allows you to easily benchmark transformer code

lib/benchmarks.dart

Index: lib/benchmarks.dart
diff --git a/lib/benchmarks.dart b/lib/benchmarks.dart
new file mode 100644
index 0000000000000000000000000000000000000000..c048457134ca82faa3ba9de81137117f51ccc308
--- /dev/null
+++ b/lib/benchmarks.dart
@@ -0,0 +1,76 @@
+// Copyright (c) 2014, 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 code_transformers.benchmarks;
+
+import 'dart:async';
+import 'package:barback/barback.dart';
+import 'src/async_benchmark_base.dart';
+
+/// A benchmark for testing the performance of transformer phases.
+class TransformerBenchmark extends AsyncBenchmarkBase {
+  /// Internal abstraction layer for barback.
+  _BenchmarkHelper _helper;
+
+  /// The transformer phases to be ran.
+  final List<List<Transformer>> transformers;
+
+  /// The files to pass to barback.
+  final Map<AssetId, String> files;
+
+  TransformerBenchmark(this.transformers, this.files);
+
+  @override
+  Future setup() {
+    _helper = new _BenchmarkHelper(transformers, files);
+    return new Future.value();
+  }
+
+  @override
+  Future run() => _helper.run();
+
+  @override
+  teardown() {
+    _helper = null;
+    return new Future.value();
+  }
+}
+
+/// Barback abstraction layer.
+class _BenchmarkHelper implements PackageProvider {
+  /// All the files available.
+  final Map<AssetId, String> _files;
+
+  /// All the packages.
+  final Iterable<String> packages;
+
+  /// Internal instance of barback.
+  Barback _barback;
+
+  /// Subscription to barback results.
+  StreamSubscription<BuildResult> resultSubscription;
+
+  _BenchmarkHelper(
+      List<List<Transformer>> transformers, Map<AssetId, String> files)
+      : _files = files,
+        packages = new Set.from(files.keys.map((assetId) => assetId.package)) {
+    _barback = new Barback(this);
+    for (var package in packages) {
+      _barback.updateTransformers(package, transformers);
+    }
+  }
+
+  /// Look up an [AssetId] in [files] and return an [Asset] for it.
+  @override
+  Future<Asset> getAsset(AssetId id) =>
+      new Future.value(new Asset.fromString(id, _files[id]));
+
+  /// Tells barback which files have changed, and thus anything that depends on
+  /// it on should be computed. Returns a [Future] that completes once some
+  /// results are received.
+  Future run([Iterable<AssetId> assetIds]) {
+    if (assetIds == null) assetIds = _files.keys;
+    _barback.updateSources(assetIds);
+    return _barback.results.first;
+  }
+}