Interface Assertions not Working in Specific Cases

I'm not sure what exactly causes this. I tried to narrow this down but, unfortunately, no matter what I tried, the narrowed down code worked as expected. However, the code that I originally found the problem in is available. The tests for a project I'm working on fail in GopherJS but work fine in standard Go. In GopherJS, it panics with the line

interface conversion: io.Writer is not io.writer: missing method Write

Somewhat complicating the matter is that the io package in question is not the standard library's io package; it's a different package that's part of the aforementioned project.

I'll keep trying to narrow it down and update if I find the cause, but I figured I'd post this with the information I have so far in case someone's able to figure it out faster.

Start by running the wdte project tests with standard Go and GopherJS, then compare the interface assertion that produces the reported panic. The issue does not identify a source file or reduced reproduction, so done means isolating the failing case and making the GopherJS result match standard Go.