How to reproduce:
1. Open Command Prompt
2. Launch Unity editor from a Command prompt(e.g "pathToUnity\Unity.exe" -projectPath "pathToAnyProject")
-- Unity editor should start and console doesn't wait as expected
3. Close Unity Editor
--Random output is written

Expected result: The Editor should either run a Windowed application (so no stdout output), or a Console application (with possible stdout output).
Actual result: The result is a mixture of Windowed application which still runs when Editor is launched but after closing Editor output is written.

Regression first introduced in: 5.5.1p2
Reproducible with: 5.5.1p2, 5.5.3p2, 5.5.4p2, 5.6.1f1, 5.6.3p4, 2017.1.0p2, 2017.1.1p3, 2017.2.0b11, 2017.3.0b2
Not reproducible with: 5.5.0a6, 5.5.1f1, 5.5.1p1