mypy fails to recognize functions as generators when `sys.platform` is used in conditions

mypy seems to be too thorough when interpreting if sys.platform == ... conditions. Consider the following file:

import sys
from typing import Generator

def f() -> Generator[int, None, None]:
    if sys.platform == "darwin":
        yield 1

I'm trying to run mypy for this file on Linux using mypy gen.py

Expected behavior:
mypy test passes for this file on Linux

Actual behavior:
I get this on Linux:

$ mypy gen.py 
gen.py:4: error: Missing return statement

At the same time, mypy on macOS does not complain. I think that this is related to #698

Platform information:

$ uname -sr; mypy --version; python --version
Linux 4.15.0-34-generic
mypy 0.630
Python 2.7.15rc1

Reproduce the report using the gen.py example and run mypy on Linux, then compare the result with macOS behavior. Trace how the checker handles sys.platform conditions and generator functions; done means the example no longer reports a missing return statement on Linux without regressing the macOS result.