Adds a repeatable --url_mapping flag to dartanalyzer

runtime/tools/create_snapshot_bin.py

 #!/usr/bin/env python
 #
 # 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.
 
 """Script to create snapshot bin file."""
 
 import getopt
 import optparse
@@ skipping 18 matching lines @@
       help="output file name into which snapshot in binary form is generated")
   result.add_option("--script",
       action="store", type="string",
       help="Dart script for which snapshot is to be generated")
   result.add_option("--package_root",
       action="store", type="string",
       help="path used to resolve package: imports.")
   result.add_option("--url_mapping",
       default=[],
       action="append",
-      help="mapping from url to file name, used when generating snapshots")
+      help=("mapping from url to file name, used when generating snapshots " +
+            "E.g.: --url_mapping=fileUri,/path/to/file.dart"))
   result.add_option("-v", "--verbose",
       help='Verbose output.',
       default=False, action="store_true")
   result.add_option("--target_os",
       action="store", type="string",
       help="Which os to run the executable on. Current choice is android")
   result.add_option("--abi",
       action="store", type="string",
       help="Desired ABI for android target OS. armeabi-v7a or x86")
   return result
@@ skipping 50 matching lines @@
     utils.RunCommand(command, outStream=sys.stderr, errStream=sys.stderr,
                      verbose=options.verbose, printErrorInfo=True)
   except Exception as e:
     return -1
 
   return 0
 
 
 if __name__ == '__main__':
   sys.exit(Main())