1. What happened
For the given project, while switching between platforms, e.g. Android and Standalone, I'm getting:
Assertion failed: Assertion failed on expression: '!GetPersistentManager().IsStreamLoaded(metaDataPath)'
Asset import did not unload metadata path. This will leak memory. File a bug with repro steps please.

This is regression from 5.6.1p1

2. How we can reproduce it using the example you attached
- Open one of Demo Scenes if it isn't opened. As otherwise the issue doesn't occur.
- Build Settings
- Switch between Android <> Standalone <> iOS
>>>
At the end of the switch the error appears.

Ths project is an unmodified A$ package 'Realistic Hovercraft Controller' https://www.assetstore.unity3d.com/en/#!/content/44792.