Reproduction steps:
1. Download "New Unity Project (13).zip" project and open in Unity
2. Observe

Expected: no errors appear in the console
Actual: 77 errors appear in the console

Reproduced on: 2018.3.0a1; 2018.3.0a4
Not reproduced on: 2018.2.0f2

Couple of the errors:
C:\Users\UnityAndroid\AppData\Local\Unity\cache\packages\packages.unity.com\com.unity.entities@0.0.12-preview.8\Unity.Entities.Properties\TypeInformation.cs(284,20): error CS0246: The type or namespace name 'PropertyBag' could not be found (are you missing a using directive or an assembly reference?)

C:\Users\UnityAndroid\AppData\Local\Unity\cache\packages\packages.unity.com\com.unity.entities@0.0.12-preview.8\Unity.Entities.Properties\TypeInformation.cs(342,34): error CS0115: 'PropertyCodeReflectionParser.EnumPrimitiveProperty<TValue>.IsReadOnly': no suitable method found to override

C:\Users\UnityAndroid\AppData\Local\Unity\cache\packages\packages.unity.com\com.unity.entities@0.0.12-preview.8\Unity.Entities.Properties\JsonVisitor.cs(152,14): error CS0538: 'ICustomVisit<float2x2>' in explicit interface declaration is not an interface