Use the absolute path when creating a stamp file

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 192 matching lines @@
     return 1
   # Calculate absolute paths before changing directory.
   if (options.package_root != None):
     options.package_root = os.path.abspath(options.package_root)
   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 (options.stamp != ''):
+    options.stamp = os.path.abspath(options.stamp)
   if len(args) == 1:
     args[0] = os.path.abspath(args[0])
   # Pub must be run from the project's root directory.
   ChangeDirectory(options.directory)
   result = ExecuteCommand(options, args)
   if result == 0:
     CreateTimestampFile(options)
   return result
 
 
 if __name__ == '__main__':
   sys.exit(main());