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

test/benchmarks_test.dart

Index: test/benchmarks_test.dart
diff --git a/test/benchmarks_test.dart b/test/benchmarks_test.dart
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+++ b/test/benchmarks_test.dart
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+// 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.test.benchmarks_test;
+
+import 'package:barback/barback.dart';
+import 'package:code_transformers/benchmarks.dart';
+import 'package:unittest/compact_vm_config.dart';
+import 'package:unittest/unittest.dart';
+
+main() {
+  useCompactVMConfiguration();
+
+  test('can benchmark transformers', () {
+    var transformer = new TestTransformer();
+    var transformers = [[transformer]];
+    var id = new AssetId('foo', 'lib/bar.dart');
+    var files = {
+        id: 'library foo.bar;',
+    };
+    var benchmark = new TransformerBenchmark(transformers, files);
+    return benchmark.measure().then((result) {
+      expect(result, greaterThan(0));
+      expect(transformer.filesSeen[id], isNotNull);
+      expect(transformer.filesSeen[id], greaterThanOrEqualTo(10));
+    });
+  });
+}
+
+class TestTransformer extends Transformer {
+  /// Map of the asset ids that were seen to how many times they were seen.
+  Map<AssetId, int> filesSeen = {};
+
+  @override
+  bool isPrimary(AssetId id) => true;
+
+  @override
+  void apply(Transform transform) {
+    filesSeen.putIfAbsent(transform.primaryInput.id, () => 0);
+    filesSeen[transform.primaryInput.id] += 1;
+  }
+}