Fix runtime error in copytree() MockFileSystem

Tools/Scripts/webkitpy/common/system/filesystem_mock.py

 # Copyright (C) 2009 Google Inc. All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #    * Redistributions of source code must retain the above copyright
 # notice, this list of conditions and the following disclaimer.
 #    * Redistributions in binary form must reproduce the above
 # copyright notice, this list of conditions and the following disclaimer
@@ skipping 373 matching lines @@
             # cases like path='/foo/b' and f='/foo/bar/baz'.
             if f == path or f.startswith(path + self.sep):
                 self.files[f] = None
 
         self.dirs = set(filter(lambda d: not (d == path or d.startswith(path + self.sep)), self.dirs))
 
     def copytree(self, source, destination):
         source = self.normpath(source)
         destination = self.normpath(destination)
 
-        for source_file in self.files:
+        for source_file in list(self.files):
             if source_file.startswith(source):
                 destination_path = self.join(destination, self.relpath(source_file, source))
                 self.maybe_make_directory(self.dirname(destination_path))
                 self.files[destination_path] = self.files[source_file]
 
     def split(self, path):
         idx = path.rfind(self.sep)
         if idx == -1:
             return ('', path)
         return (path[:idx], path[(idx + 1):])
@@ skipping 71 matching lines @@
         return self.data.readline(length)
 
     def __iter__(self):
         return self.data.__iter__()
 
     def next(self):
         return self.data.next()
 
     def seek(self, offset, whence=os.SEEK_SET):
         self.data.seek(offset, whence)