Fix runtime error in copytree() MockFileSystem

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

 # vim: set fileencoding=utf-8 :
 # Copyright (C) 2010 Google Inc. All rights reserved.
 #
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 # modification, are permitted provided that the following conditions are
 # met:
 #
 #    * Redistributions of source code must retain the above copyright
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 #    * Redistributions in binary form must reproduce the above
@@ skipping 100 matching lines @@
 
     def test_rmtree(self):
         self.fs.chdir(self.generic_test_dir)
         self.fs.rmtree('foo')
         self.assertTrue(self.fs.exists('foodir'))
         self.assertTrue(self.fs.exists(self.fs.join('foodir', 'baz')))
         self.fs.rmtree('foodir')
         self.assertFalse(self.fs.exists('foodir'))
         self.assertFalse(self.fs.exists(self.fs.join('foodir', 'baz')))
 
+    def test_copytree(self):
+        self.fs.chdir(self.generic_test_dir)
+        self.fs.copytree('foodir/', 'bardir/')
+        self.assertTrue(self.fs.exists('bardir'))
+        self.assertTrue(self.fs.exists(self.fs.join('bardir', 'baz')))
 
 class RealFileSystemTest(unittest.TestCase, GenericFileSystemTests):
     def setUp(self):
         self.fs = FileSystem()
         self.setup_generic_test_dir()
 
         self._this_dir = os.path.dirname(os.path.abspath(__file__))
         self._missing_file = os.path.join(self._this_dir, 'missing_file.py')
         self._this_file = os.path.join(self._this_dir, 'filesystem_unittest.py')
 
@@ skipping 170 matching lines @@
         fs = FileSystem()
         self.assertTrue(fs.remove('filename', remove_with_exception))
         self.assertEqual(-1, RealFileSystemTest._remove_failures)
 
     def test_sep(self):
         fs = FileSystem()
 
         self.assertEqual(fs.sep, os.sep)
         self.assertEqual(fs.join("foo", "bar"),
                           os.path.join("foo", "bar"))