To reproduce:

1. Create a new project - set to Old mono runtime
2. Open activity monitor
3. Play/stop scene multiple times

Expected: memory usage should be the same each time scene is played
Actual: memory usage increases each time scene is played

Note:
With unsaved scene:
on 5.3, 5.4, 5.5 increases ~10MB
on 5.6, 2017.1.0 ~20MB
When scene is saved:
on 5.4, 5.5 increases ~1-10MB
on 5.6, 2017.1.0 ~5-12MB

Reproduced on: 5.4.5p2, 5.5.0f1, 5.5.3p3, 5.6.0b4, 5.6.1f1, 2017.1.0b1, 2017.1.0b5

Tested on OSX 10.12.4