Steps to repro:

1. Open Unity;
2. Crete new project;
3. Open Asset Store (Window - Asset Store);
4. Search for 'City Building Perfect Kit' (https://www.assetstore.unity3d.com/en/#!/content/50027), download and import it to the project;
5. In the 'Project' folder go to Assets -> CityBuildingPerfectKit -> Scenes;
6. Open 'Town' scene;
7. Observe the surface mesh rendering.

Actual result:
Shadowy artifact appears on surface mesh after importing asset and opening the scene (see attached gif video and actual.png).
Regression introduced somewhere in Unity 5.4.

Workaround is to save scene/project and reopen Editor - in this case artifact disappears.

Notes:
- Reproducible in: 5.4.0b17, 5.4.0b16, 5.4.0b8;
- Not reproducible in 5.3.4p4, 5.3.3p2;
- Reproducible on both OSX 10.11.4 and on Windows 10;
- Reproducible ONLY when importing abovementioned package from Asset Store, doesn't repro when importing package, created from downloaded asset;
- Reproduces in both Editor's Scene/Game view and in Standalone runtime build.