This also happens with Unexpected token ‘:’ when the file exists, but isn’t the right file, as described below.
I thought I had downloaded copies of draco_decoder.wasm and draco_wasm_wrapper.js from github, but used the wrong url and ended up with two small text files with the correct filenames, but each containing “404: Not Found”. My javascript console in Firefox’s web developer tools kept telling me it found the files, but still didn’t work. After getting the correct files in place, problem solved.