Reproduction steps:
1. Open the attached project "EntityTest.zip";
2. Open the SampleScene;
3. Enter a Play mode.

Actual results: the ReflectionTypeLoadException is thrown:
ReflectionTypeLoadException: Exception of type 'System.Reflection.ReflectionTypeLoadException' was thrown.
System.Reflection.Assembly.GetTypes () (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/Assembly.cs:410)
Unity.Entities.DefaultWorldInitialization.Initialize (System.String worldName, System.Boolean editorWorld) (at /Users/qa/Library/Unity/cache/packages/packages.unity.com/com.unity.entities@0.0.12-preview.2/Unity.Entities.Hybrid/Injection/DefaultWorldInitialization.cs:69)
Unity.Entities.AutomaticWorldBootstrap.Initialize () (at /Users/qa/Library/Unity/cache/packages/packages.unity.com/com.unity.entities@0.0.12-preview.2/Unity.Entities.Hybrid/Injection/AutomaticWorldBootstrap.cs:11)

Reproduced on: 2018.1.0b9, 2018.1.2f1, 2018.2.0b6.
Not reproduced on: 2018.1.0b8 and lower.