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

lib/src/async_benchmark_base.dart

Index: lib/src/async_benchmark_base.dart
diff --git a/lib/src/async_benchmark_base.dart b/lib/src/async_benchmark_base.dart
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+++ b/lib/src/async_benchmark_base.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.src.async_benchmark_base;
+
+import 'dart:async';
+
+/// An adaptation of [BenchmarkBase] from the `benchmark_harness` package that
+/// works for async benchmarks.
+/// TODO(jakemac): Get this merged into `benchmark_harness`.
+class AsyncBenchmarkBase {
+  // Empty constructor.
+  const AsyncBenchmarkBase();
+
+  // The benchmark code.
+  // This function is not used, if both [warmup] and [exercise] are overwritten.
+  Future run() => new Future.value();
+
+  // Runs a short version of the benchmark. By default invokes [run] once.
+  Future warmup() => run();
+
+  // Exercices the benchmark. By default invokes [run] 10 times.
+  Future exercise({int iterations: 10}) {
+    var i = 0;
+    return Future.doWhile(() {
+      if (i >= iterations) return new Future.value(false);
+      i++;
+      return run().then((_) => true);
+    });
+  }
+
+  // Not measured setup code executed prior to the benchmark runs.
+  Future setup() => new Future.value();
+
+  // Not measures teardown code executed after the benchark runs.
+  Future teardown() => new Future.value();
+
+  // Measures the score for this benchmark by executing it repeatedly until
+  // time minimum has been reached.
+  static Future<double> measureFor(Function f, int minimumMillis) {
+    int minimumMicros = minimumMillis * 1000;
+    int iter = 0;
+    Stopwatch watch = new Stopwatch();
+    watch.start();
+    int elapsed = 0;
+    return Future.doWhile(() {
+      if (elapsed > minimumMicros) return new Future.value(false);
+      return f().then((_) {
+        elapsed = watch.elapsedMicroseconds;
+        iter++;
+        return true;
+      });
+    }).then((_) => elapsed / iter);
+  }
+
+  // Measures the average time to call `run` once and returns it.
+  Future<double> measure({int iterations: 10}) {
+    // Unmeasured setup code.
+    return setup().then((_) {
+      // Warmup for at least 100ms. Discard result.
+      return measureFor(() => warmup(), 100);
+    }).then((_) {
+      // Run the benchmark for at least 2000ms.
+      return measureFor(() => exercise(iterations: iterations), 2000);
+    }).then((result) {
+      // Tear down the test (unmeasured) and return the result divided by the
+      // number of iterations.
+      return teardown().then((_) => result / iterations);
+    });
+  }
+}