Cannot call dict.get with a falsy second argument and an 'or' expression.

In mypy 0.610, as well as the latest version from master, the following throws an error under Python 3.6

def foo(a: str):
    pass


d = {'key': 'a'}
foo(d.get('key', None) or '26')

test.py:6: error: Argument 2 to "get" of "Mapping" has incompatible type "None"; expected "str"

However, no matter what, foo will still receive a str, given that None is falsy. Therefore, no error should be thrown. This occurs with just a simple call to mypy foo.py with no flags.

Start with the minimal reproduction in test.py and run it with mypy foo.py under Python 3.6. Trace how the type checker handles Mapping.get when its result is used in the shown or expression. Done means the example no longer reports an incompatible None argument while preserving appropriate type checking for similar calls.