--- a/dom/bindings/mozwebidlcodegen/test/test_mozwebidlcodegen.py
+++ b/dom/bindings/mozwebidlcodegen/test/test_mozwebidlcodegen.py
@@ -244,39 +244,30 @@ class TestWebIDLCodegenManager(unittest.
             fh.write('# Original content')
             fh.flush()
             mod = imp.load_source('mozwebidlcodegen.fakemodule', fh.name)
             mod.__file__ = fake_path
 
             args = self._get_manager_args()
             m1 = WebIDLCodegenManager(**args)
             with MockedOpen({fake_path: '# Original content'}):
-                old_exists = os.path.exists
                 try:
-                    def exists(p):
-                        if p == fake_path:
-                            return True
-                        return old_exists(p)
-
-                    os.path.exists = exists
-
                     result = m1.generate_build_files()
                     l = len(result.inputs)
 
                     with open(fake_path, 'wt') as fh:
                         fh.write('# Modified content')
 
                     m2 = WebIDLCodegenManager(**args)
                     result = m2.generate_build_files()
                     self.assertEqual(len(result.inputs), l)
 
                     result = m2.generate_build_files()
                     self.assertEqual(len(result.inputs), 0)
                 finally:
-                    os.path.exists = old_exists
                     del sys.modules['mozwebidlcodegen.fakemodule']
 
     def test_copy_input(self):
         """Ensure a copied .webidl file is handled properly."""
 
         # This test simulates changing the type of a WebIDL from static to
         # preprocessed. In that scenario, the original file still exists but
         # it should no longer be consulted during codegen.