How to reproduce:
1. Open the attached project "1181838.zip"
2. Enable Autoconnect profiler in the project's build settings
3. Build the project
4. Select any frame

Expected result: Calculations are done in the main thread and worker threads and take up hundreds of milliseconds to finish
Actual result: Almost nothing is done in the main thread and no worker threads are being used

Reproducible with: Unity 2019.2.17f1 (Entities 0.1.1)

Notes:
Reproducible on Windows and MacOS IL2CPP builds
The issue is not reproducible on Unity 2018.4.15f1 and below, as the latest available Entities package version is 0.0.12-preview.24 which misses required functionality.
The issue does not reproduce on Unity 2019.3.0a9 and below because UnityLinker cannot be found.
On Unity 2019.3.0a10 (Entities 0.1.1) and above JobComponentSystem works as expected.