emscripten_async_wget2_data hangs instead of loading file

I have created a [minimal test](https://gist.github.com/arielm/11567c883d1f0662eaec56b1cb3e0b0e) (based on `tests/test_emscripten_async_wget2_data.cpp`) which tries to load a text file.

It works with a small text file (few bytes), but it hangs the browser when loading a ~500KB file.

Note that it doesn't matter if I build using ASYNCIFY or not.