[Closure Externs] Fix presubmit check

extensions/common/api/externs_checker_test.py

 #!/usr/bin/env python
 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import os
 import sys
 import unittest
 
 from externs_checker import ExternsChecker
 
 sys.path.append(
     os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', '..', '..'))
 
-from PRESUBMIT_test_mocks import MockInputApi, MockOutputApi, MockFile
+from PRESUBMIT_test_mocks import (MockInputApi, MockOutputApi, MockFile,
+                                  MockChange)
 
 
 class ExternsCheckerTest(unittest.TestCase):
   API_PAIRS = {'a': '1', 'b': '2', 'c': '3'}
 
   def _runChecks(self, files, exists=lambda f: True):
     input_api = MockInputApi()
     input_api.os_path.exists = exists
     input_api.files = [MockFile(f, '') for f in files]
+    input_api.change = MockChange(input_api.files)
     output_api = MockOutputApi()
     checker = ExternsChecker(input_api, output_api, self.API_PAIRS)
     return checker.RunChecks()
 
   def testModifiedSourceWithoutModifiedExtern(self):
     results = self._runChecks(['b', 'test', 'random'])
     self.assertEquals(1, len(results))
     self.assertEquals(1, len(results[0].items))
     self.assertEquals('b', results[0].items[0])
     self.assertEquals(
@@ skipping 26 matching lines @@
 
   def testApiFileDoesNotExist(self):
     exists = lambda f: f in ['a', 'b', 'c', '1', '2']
     with self.assertRaises(OSError) as e:
       self._runChecks(['a'], exists)
     self.assertEqual('Path Not Found: 3', str(e.exception))
 
 
 if __name__ == '__main__':
   unittest.main()