Steps to reproduce:
1. Open an empty project
2. Import user-attached package "MRTK-Test.unitypackage" (rmb in Projects panel->Import Package->Custom Package)
3. See errors in the Console

Reproduced with: 2018.1.4f1, 2018.2.0b9, 2018.3.0a3

Expected result: Assembly Definitions are compiled successfully and thus the user gets a prompt to switch to .NET4.X
Actual result: Assembly Definitions are not compiled successfully and the user only gets to see API incompatibility errors in the Console. Reloading the project makes the prompt appear as intended

This behavior is assumed to be incorrect in accordance with what is written in the Release Notes of Unity 2018.1:
Editor: Unity now compiles Assembly Definition File (asmdef) assemblies on startup, before any other scripts (such as Assembly-CSharp.dll), and does not stop compilation on the first compile error. Uniy loads all asmdef assemblies that succesfully compile and have all their references compiled, then compiles the remaining scripts (Assembly-CSharp.dll and friends). This ensures that Unity packages are always built and and loaded, regardless of other compile errors in the Project.