Upgrade project problem - this project works fine in 5.6.0p2

On importing "Tanks!!! Reference Project" project from A$ to 2017.1 - errors appear.

How we can reproduce it
1 Create ne project
2 Go to AssetStore and find "Tanks!!! Reference Project" (https://www.assetstore.unity3d.com/en/#!/content/80165)
3 Import this package to your project

Actual result: when importing will be finished you will see 4 errors:
- "A polygon of TankTurret is self-intersecting and has been discarded. Try cleaning the mesh before exporting."
- "A polygon of Cliff3 is self-intersecting and has been discarded. Try cleaning the mesh before exporting."
- "A polygon of Column02 Shadow is self-intersecting and has been discarded. Try cleaning the mesh before exporting."
- "A polygon of Column02 is self-intersecting and has been discarded. Try cleaning the mesh before exporting."

Expected results: there should not be any errors