func AbsFile(dir, file, rewrites string) string {
	abs := file
	if dir != "" && !filepath.IsAbs(file) {
		abs = filepath.Join(dir, file)
	}

	abs, rewritten := ApplyRewrites(abs, rewrites)
	if !rewritten && buildcfg.GOROOT != "" && hasPathPrefix(abs, buildcfg.GOROOT) {
		abs = "$GOROOT" + abs[len(buildcfg.GOROOT):]
	}

	// Rewrite paths to match the slash convention of the target.