When I was working on the sourcemap and playground, I found that sometimes gopherjs serve couldn't find a file to serve.

For example, serving from github.com/[...]/gopherjs.github.io/playground/ would fail to find the internal/playground package (that package is in some of the ideas I've had for the playground). The requested file was github.com/[...]/gopherjs.github.io/playground/internal/playground. That file was not able to be found. I added another basic import call but would get no required module provides package [...]; to add it: go get [...]. So I added some other ways to quickly look up files without using import.

I also:

• added some more logging for the serve so that we can see what and where it is finding these files if we set --log_level debug
• added a method to create a sourcemapx.Filter that can be used with the playground
• updated substr, that is deprecated, with substring. They are not one-to-one (see the substring link for details), but since we mostly use it to trim the start, with one exception that starts at 0, the difference between substr(start, length) and substring(start, end) looks like a drop in replacement for our code