go/build: support Import of local import path in standard library for gccgo

It's possible for a local import path to refer to a standard library
package. This was not being correctly handled for gccgo. When using
gccgo, change the code to permit the existing lexical test, and to
accept a missing directory for a standard package found via a local
impor path.

Change-Id: Ia9829e55c0ff62e7d1f01a1d6dc9fcff521501ca
Reviewed-on: https://go-review.googlesource.com/137439
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/go/build/build.go

@@ -544,7 +544,7 @@ func (ctxt *Context) Import(path string, srcDir string, mode ImportMode) (*Packa
 		inTestdata := func(sub string) bool {
 			return strings.Contains(sub, "/testdata/") || strings.HasSuffix(sub, "/testdata") || strings.HasPrefix(sub, "testdata/") || sub == "testdata"
 		}
-		if ctxt.GOROOT != "" && ctxt.Compiler != "gccgo" {
+		if ctxt.GOROOT != "" {
 			root := ctxt.joinPath(ctxt.GOROOT, "src")
 			if sub, ok := ctxt.hasSubdir(root, p.Dir); ok && !inTestdata(sub) {
 				p.Goroot = true
@@ -715,6 +715,11 @@ Found:
 	// non-nil *Package returned when an error occurs.
 	// We need to do this before we return early on FindOnly flag.
 	if IsLocalImport(path) && !ctxt.isDir(p.Dir) {
+		if ctxt.Compiler == "gccgo" && p.Goroot {
+			// gccgo has no sources for GOROOT packages.
+			return p, nil
+		}
+
 		// package was not found
 		return p, fmt.Errorf("cannot find package %q in:\n\t%s", path, p.Dir)
 	}