InFile.load_from_files should try to load only '.in' files

Source/build/scripts/in_file_unittest.py

 #!/usr/bin/env python
 # Copyright (C) 2013 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
@@ skipping 54 matching lines @@
             'namespace': '',
             'fruit': False,
         }
         in_file = InFile(lines, defaults, default_parameters)
         expected_parameters = {
             'namespace': 'TestNamespace',
             'fruit': True,
         }
         self.assertEquals(in_file.parameters, expected_parameters)
 
+    def test_assertion_for_non_in_files(self):
+        in_files = ['some_sample_file.json']
+        assertion_thrown = False
+        try:
+            in_file = InFile.load_from_files(in_files, None, None, None)
+        except AssertionError:
+            assertion_thrown = True
+        except:
+            pass
+        self.assertTrue(assertion_thrown)
+
 
 if __name__ == "__main__":
     unittest.main()