net/http: support request cancellation on js/wasm

[dmitshur]

We should implement HTTP request cancellation via request context cancellation on js/wasm now that AbortSignal has become part of the DOM living standard and many major browsers have implemented it.

Also see https://stackoverflow.com/a/47250621/622447 for more relevant information.

/cc @neelance @johanbrandhorst @bradfitz

[dmitshur]

This is already implemented! 🎉

It was implemented 2 years ago in the original CL 114515. See here and here. I just missed it on my first look through the code.

[gopherbot]

Change https://golang.org/cl/226204 mentions this issue: net/http: release callbacks after fetch promise completes