Parses crash on invalid use of unicode string literal

[JukkaL]

This causes a parser crash (Python 3 mode):

x u''

Here's the stack trace:

Traceback (most recent call last):
  File "scripts/mypy", line 169, in <module>
    main()
  File "scripts/mypy", line 34, in main
    type_check_only(path, module, bin_dir, options)
  File "scripts/mypy", line 78, in type_check_only
    python_path=options.python_path)
  File "/home/jukka/project/mypy/mypy/build.py", line 164, in build
    result = manager.process(UnprocessedFile(info, program_text))
  File "/home/jukka/project/mypy/mypy/build.py", line 342, in process
    next.process()
  File "/home/jukka/project/mypy/mypy/build.py", line 660, in process
    tree = self.parse(self.program_text, self.path)
  File "/home/jukka/project/mypy/mypy/build.py", line 739, in parse
    custom_typing_module=self.manager.custom_typing_module)
  File "/home/jukka/project/mypy/mypy/parse.py", line 80, in parse
    tree = parser.parse(s)
  File "/home/jukka/project/mypy/mypy/parse.py", line 117, in parse
    file = self.parse_file()
  File "/home/jukka/project/mypy/mypy/parse.py", line 125, in parse_file
    defs = self.parse_defs()
  File "/home/jukka/project/mypy/mypy/parse.py", line 250, in parse_defs
    self.expect_break()
  File "/home/jukka/project/mypy/mypy/parse.py", line 1535, in expect_break
    return self.expect_type(Break)
  File "/home/jukka/project/mypy/mypy/parse.py", line 1529, in expect_type
    self.parse_error()
  File "/home/jukka/project/mypy/mypy/parse.py", line 1547, in parse_error
    self.parse_error_at(self.current())
  File "/home/jukka/project/mypy/mypy/parse.py", line 1558, in parse_error_at
    msg = 'Parse error before {}'.format(token_repr(tok))
  File "/home/jukka/project/mypy/mypy/parse.py", line 1674, in token_repr
    raise ValueError('Unknown token {}'.format(repr(tok)))
ValueError: Unknown token UnicodeLit( u'')