Print the correct path to dartfmt in the presubmit error message.

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.
 """
 
@@ skipping 74 matching lines @@
         if old_version_has_errors:
           print("WARNING: %s has existing and possibly new dartfmt issues" %
             git_file.LocalPath())
         else:
           unformatted_files.append(filename)
 
   if unformatted_files:
     return [output_api.PresubmitError(
         'File output does not match dartfmt.\n'
         'Fix these issues with:\n'
-        'dartfmt -w \\\n%s' % ' \\\n'.join(unformatted_files))]
+        '%s -w \\\n%s' % (prebuilt_dartfmt, ' \\\n'.join(unformatted_files)))]
 
   return []
 
 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)