Canary doesn't load dwarf symbols with "-g4 --source-map-base"

I don't know if it's a browser (Canary) or an emscripten issue:

On any [trivial example](https://emscripten.org/docs/getting_started/Tutorial.html):
- When compiling and linking with "-g4 --source-map-base http://0.0.0.0:8000/", Canary correctly loads the .wasm.map file, and finds the source files, but debugging symbols are not resolved (local variables, arguments, etc.)

Other cases work fine:
- When compiling and linking with "-g", you get the dwarf symbols, but files are accessed through the local filesystem.
- When compiling and linking with "-gseparate-dwarf", you get the separate dwarf symbols, but files are accessed through the local filesystem.

Tested version: Emscripten 2.0.15 (Mar 09, 2021) (macOS)