Update README.md and HACKING.md and resulting yak shave.

sky/tools/test_perf

 #!/usr/bin/env python
 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 from skypy.find_tests import find_tests
 from skypy.paths import Paths
 import argparse
 import os
 import re
 import requests
 import skypy.configuration as configuration
 import skypy.skyserver
 import subprocess
 
 
 SUPPORTED_MIME_TYPES = [
-    'text/html',
     'text/sky',
-    'text/plain',
+    'application/dart',
 ]
 HTTP_PORT = 9999
 URL_ROOT = 'http://localhost:%s/' % HTTP_PORT
 DASHBOARD_URL = 'https://chromeperf.appspot.com/add_point'
 BENCHMARKS_DIR = 'benchmarks'
 
 
 def values_from_output(output):
     # Parse out the raw values from the PerfRunner output:
     #   values 90, 89, 93 ms
@@ skipping 91 matching lines @@
     try:
         harness.main()
     except (KeyboardInterrupt, SystemExit):
         pass
     finally:
         harness.shutdown()
 
 
 if __name__ == '__main__':
     main()