Repro steps:
- download the Skeleton_GoodScript.unitypackage
- start a new proj with 2018.3.0b5 or trunk
- import this package to the new proj
- you'll notice that the compiler will complain about some dlls:

Microsoft (R) Visual C# Compiler version 2.7.0.65535 (ced8f62d)
Copyright (C) Microsoft Corporation. All rights reserved.

error CS1703: Multiple assemblies with equivalent identity have been imported: 'C:\prototypes\66\Assets\uMMORPG\Plugins\SQLite\sqlite_common\System.Data.dll' and 'C:\Program Files\Unity 2018.3.0a9\Editor\Data\NetStandard\compat\2.0.0\shims\netfx\System.Data.dll'. Remove one of the duplicate references.

- switch to .NET 3.5 (Legacy) to get rid of those errors
- look at the Demo scene - there's a Skeleton there -> the skeleton would have lost all its prefab references and there will be navmesh errors in the console

Workaround: re-import the prefab