crash when type checking a `break` outside loop

[rwbarton]

On the program

break

mypy crashes with the error

Traceback (most recent call last):
  File "/Users/rbarton/mypy/scripts/mypy", line 6, in <module>
    main(__file__)
[...]
  File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/mypy/checker.py", line 2028, in visit_break_stmt
    self.binder.allow_jump(self.binder.loop_frames[-1] - 1)
IndexError: list index out of range

*** INTERNAL ERROR ***

break.py:1: error: Internal error -- please report a bug at https://github.com/python/mypy/issues

NOTE: you can use "mypy --pdb ..." to drop into the debugger when this happens.

Semantic analysis actually identifies that there is a break outside a loop and "fails", but this failure isn't fatal and mypy goes on to type check the module and crashes before printing the error. Not sure what the cleanest fix is.

[gvanrossum]

Can't we just make that a fatal error?

[JukkaL]

Making it a fatal error sounds reasonable.