Fix other.test_warn_undefined on Windows.

Restores code that was commented out in https://github.com/kripken/emscripten/commit/424b709cac07c3735007b2c31c24caed5dcedc24#diff-b0b85d67c2201828828422795fb44581R311 , fixes the test `other.test_warn_undefined` on Windows.

Tested on Windows that node.js still suffers the flush bug in latest 0.10.31, so the workaround is still needed.