Add presubmit check that runs dartfmt

PRESUBMIT.py

 # Copyright (c) 2012, 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.
 
 """Top-level presubmit script for Dart.
 
 See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts
 for more details about the presubmit API built into gcl.
 """
 
-def CheckChangeOnCommit(input_api, output_api):
+import imp
+import os
+
+def _CheckBuildStatus(input_api, output_api):
   results = []
   status_check = input_api.canned_checks.CheckTreeIsOpen(
       input_api,
       output_api,
       json_url='http://dart-status.appspot.com/current?format=json')
   results.extend(status_check)
   return results
+
+def _CheckDartFormat(input_api, output_api):
+  utils = imp.load_source('utils',
+      os.path.join(input_api.change.RepositoryRoot(), 'tools', 'utils.py'))
+
+  results = []
+  prebuilt_dartfmt = os.path.join(utils.CheckedInSdkPath(), 'bin', 'dartfmt')

[zra, 2017/03/20 19:26:06]

The presubmit check should degrade gracefully if the this file doesn't exist. It
doesn't exist, e.g. in the Fuchsia tree.

[Jacob, 2017/03/20 19:59:07]

Done. I now print a warning if the file doesn't exist.

+
+  for git_file in input_api.AffectedTextFiles():
+    filename = git_file.AbsoluteLocalPath()
+    if filename.endswith('.dart'):
+      cmd = '%s --set-exit-if-changed -n %s ' % (prebuilt_dartfmt, filename)
+      if os.system(cmd):

[zra, 2017/03/20 19:26:06]

Could you rather use the presubmit API to run commands? See:
http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts for

[Jacob, 2017/03/20 19:59:07]

Done. Thanks for pointing out the existing presubmit API that does what I need.

+            results += [output_api.PresubmitError(
+            'Must run dartfmt on %s' % filename)]
+
+  return results
+
+def CheckChangeOnCommit(input_api, output_api):
+  return (_CheckBuildStatus(input_api, output_api) +
+          _CheckDartFormat(input_api, output_api))
+
+def CheckChangeOnUpload(input_api, output_api):
+  return _CheckDartFormat(input_api, output_api)