Fix unnecessary Observatory rebuilds in Flutter

tools/observatory_tool.py

 #!/usr/bin/env python
 # Copyright (c) 2015, 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.
 """Helper for building and deploying Observatory"""
 
 import argparse
 import os
 import platform
 import shutil
@@ skipping 11 matching lines @@
     '*.scriptUrls',
     '*.precompiled.js',
     'main.*',
     'unittest*',
     '*_buildLogs*',
     '*.log',
     '*~')
 
 usage = """observatory_tool.py [options]"""
 
+def CreateTimestampFile(options):
+  if options.stamp != '':
+    dir_name = os.path.dirname(options.stamp)
+    if dir_name != '':
+      if not os.path.exists(dir_name):
+        os.mkdir(dir_name)
+    open(options.stamp, 'w').close()
+
 def BuildArguments():
   result = argparse.ArgumentParser(usage=usage)
   result.add_argument("--package-root", help="package root", default=None)
   result.add_argument("--dart-executable", help="dart executable", default=None)
   result.add_argument("--pub-executable", help="pub executable", default=None)
   result.add_argument("--directory", help="observatory root", default=None)
   result.add_argument("--command", help="[get, build, deploy]", default=None)
   result.add_argument("--silent", help="silence all output", default=None)
   result.add_argument("--sdk", help="Use prebuilt sdk", default=None)
+  result.add_argument("--stamp", help="Write a stamp file", default='')
   return result
 
 def ProcessOptions(options, args):
   # Fix broken boolean parsing in argparse, where False ends up being True.
   if (options.silent is not None) and (options.silent == "True"):
     options.silent = True
   elif (options.silent is None) or (options.silent == "False"):
     options.silent = False
   else:
     print "--silent expects 'True' or 'False' argument."
@@ skipping 147 matching lines @@
   if (options.dart_executable != None):
     options.dart_executable = os.path.abspath(options.dart_executable)
   if (options.pub_executable != None):
     options.pub_executable = os.path.abspath(options.pub_executable)
   if (options.pub_snapshot != None):
     options.pub_snapshot = os.path.abspath(options.pub_snapshot)
   if len(args) == 1:
     args[0] = os.path.abspath(args[0])
   # Pub must be run from the project's root directory.
   ChangeDirectory(options.directory)
-  return ExecuteCommand(options, args)
+  result = ExecuteCommand(options, args)
+  if result == 0:
+    CreateTimestampFile(options)
+  return result
+
 
 if __name__ == '__main__':
   sys.exit(main());