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

gen.py の例を使って報告を再現し、Linux で mypy を実行してから、その結果を macOS の挙動と比較します。checker が sys.platform の条件とジェネレーター関数をどのように処理するかを追跡します。Linux で例が return 文の欠落を報告しなくなり、macOS の結果にリグレッションが発生しなければ完了です。