Roll Observatory packages and add a roll script

packages/code_transformers/lib/src/async_benchmark_base.dart

 // 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`.
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     }).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);
     });
   }
 }