This is a go1.9 specific issue. Creating an issue so I can elaborate on the details. I have a fix that I'll apply to go1.9 branch soon.

In Go 1.9, the crypto/x509 package, on macOS, now imports os/user package:

https://github.com/golang/go/blob/go1.9rc1/src/crypto/x509/root_darwin.go#L19

It didn't do so in Go 1.8:

https://github.com/golang/go/blob/go1.8.3/src/crypto/x509/root_darwin.go#L18

This causes GopherJS on the go1.9 branch at commit cca79e8 to fail when building net/http with:

$ gopherjs test net/http
os/user: importing "C" is not supported by GopherJS

We don't run net/http tests in CI right now, because even though the HTTP client works, there are too many tests in HTTP that rely on the server also working, which isn't supported in GopherJS. So we'd have to disable many tests. Maybe we should add a gopherjs build net/http line instead, to ensure that the package at least builds.

However, that's not the reason CI didn't catch it. The real reason is that this is a macOS-specific issue, it doesn't occur on Linux, because on Linux, crypto/x509 still doesn't import os/user package. The import is in root_darwin.go, which is constrained to GOOS=darwin because of _darwin suffix in filename.

This issue demonstrates a pretty significant limitation of the current compiler/natives mechanism to override standard library packages. We're able to override individual identifiers, but if the package happens to import some (no longer needed) packages, that can't be stopped. /cc @neelance However, dealing with that is out of scope and can be considered later.

For now, my fix will be to get os/user package to build without errors.