Avoid regenerating unchanged bindings files for Dartium.

tools/dom/scripts/multiemitter.py

 #!/usr/bin/python
 # Copyright (c) 2011, 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.
 
 """Templating to help generate structured text."""
 
 import os
 import sys
 import subprocess
@@ skipping 59 matching lines @@
 
 def _WriteFile(path, lines):
   (dir, file) = os.path.split(path)
 
   # Ensure dir exists.
   if dir:
     if not os.path.isdir(dir):
       _logger.info('Mkdir - %s' % dir)
       os.makedirs(dir)
 
-  # Remove file if pre-existing.
+  # If file exists and is unchanged, return.
+  new_contents = ''.join(lines)
   if os.path.exists(path):
-    _logger.info('Removing - %s' % path)
-    os.remove(path)
-    if os.path.exists(path):
-      _logger.info('Warning: File still exists- %s' % path)
+    with open(path) as fd:
+      contents = fd.read()
+    if new_contents == contents:
+      _logger.info('Unchanged file %s' % path)
+      return
 
   # Write the file.
   num_attempts = 4
   for i in range(num_attempts):
     try:
       _logger.info('Writing (attempt %d) - %s' % (i + 1, path))
-      f = open(path, 'w')
-      f.writelines(lines)
-      f.close()
+      with open(path, 'w') as fd:
+        fd.write(new_contents)
       return
     except IOError as error:
       last_attempt = (i == (num_attempts - 1))
       if not last_attempt:
         # Sleep for 50 ms and try again
         time.sleep(0.05)
       else:
         # FIXME(kustermann): Remove this later on.
         # We try to get more debugging information to figure out why we
         # sometimes get a "Permission denied" error when opening the file for
         # writing. (hypothesis: Another process has already opened the file.)
         _logger.info('Got exception (%s) ' % error)
 
         if sys.platform  == 'win32':
           handle_file = r'E:\handle.exe'
           if os.path.exists(handle_file):
             _logger.info('Running handle.exe for debugging purposes')
             subprocess.call([handle_file, '-a', r'E:\b\build\slave'])
           else:
             _logger.info("Couldn't find %s. Not printing open handles."
                          % handle_file)
         raise error
-