A generic async method does not return to its caller after it finishes execution on WebGL platform

To reproduce:

1. Download attached project "BugReportProject.zip" and open in Unity
2. Open "TestScene" scene
3. Enter to Play mode
Note: Observe that these messages appear on Console:

Just a dummy log
This line will be executed
This line will not execute
Dummy

4. Exit Play mode and Build&Run project on WebGL
5. When Built Player appears on your browser, open its Console

Expected result: In Browser Console and in Editor Log you will see these messages:

Just a dummy log
This line will be executed
This line will not execute
Dummy

Actual result: In Browser Console you will see only these messages:

Just a dummy log
This line will be executed

And in Editor Log you will see these messages:

Just a dummy log
This line will be executed
This line will not execute
Dummy

Reproduced on Unity 2017.1.0b4, 2017.1.0b7 and 2017.1.0b8