To reproduce:
Part I: Bundle making
1) Download and import below attached Unitypackage which contains the bundle to build
2) Make a quick Asset Bundle building script (used the example in https://docs.unity3d.com/ScriptReference/BuildPipeline.BuildAssetBundles.html ), though set it for WebGL and add different compression option
3) Build the AB (it should already be set up for 'mountain')
4) Find the built mountain.unity3d bundle, and remove .unity3d from the file (so it would be 'mountain' without any extension)
5) Put it somewhere where you will be getting it from later

Part II: Project building and testing
6) Open user attached project and scene
7) In the scene, make sure EnvironmentHandler's BaseURL points to the folder (not AB itself) where you put the bundle
8) Build it for WebGL
9) Run it from where you can access the AB
10) Click the 'Mountain' button and wait until the landscape loads
11) Repeat step 10 some more times (arbitrary, can be from 2 to 20)

Expected: The bundle is unloaded and reloaded each time properly
Actual: After some attempts, the player crashes

Reproduced in 5.4.1f1, 5.4.4p1, 5.5.0f3, 5.5.0p4
Can't test in 5.6 betas due to the blocking issues

Tested on Chrome 55.0.2883.87 m (64-bit)