As documented at https://godoc.org/golang.org/x/tools/cmd/godoc:

The presentation mode of web pages served by godoc can be controlled with the "m" URL parameter;

This works for most packages, e.g., https://golang.org/pkg/path/?m=src shows the original source code rather than the extracted documentation.

It does not work for the fake package builtin: https://golang.org/pkg/builtin/?m=src has no effect.

This is because that package needs a few of the presentation modes to be applied at all times, but this is done in a way that clears the user-provided modes:

mode := h.p.GetPageInfoMode(r)
if relpath == builtinPkgPath {
	// the fake built-in package contains unexported identifiers
	mode = NoFiltering | NoTypeAssoc
}

The fix is one character:

-mode = NoFiltering | NoTypeAssoc
+mode |= NoFiltering | NoTypeAssoc