How to reproduce:
1. Open the attached project
2. Open the "EchoWire" scene.
3. Press Play, play around a bit.
4. Open the "EchoWireBehaviour" script
5. Edit some lines and swap back to Unity
6. Repeat 3-5 until Unity hangs (sometimes takes a few times)

Actual result: Unity freezes.

Reproducible with: 5.6.3p4, 2017.1.1p3, 2017.2.0f1, 2017.3.0b2.
Not reproducible with: 2017.4.13f1, 2018.1.9f2, 2018.2.14f1, 2018.3.0b7, 2019.1.0a6.

Notes:
Freeze is caused by lines 140-152 in the "EchoWireBehaviour" script.
Could not test with 5.5 because of errors.