gc: don't emit pkgpath for error type.

Fixes #2660.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5557060
diff --git a/src/pkg/reflect/all_test.go b/src/pkg/reflect/all_test.go
index 2f9f83f..63b127d 100644
--- a/src/pkg/reflect/all_test.go
+++ b/src/pkg/reflect/all_test.go
@@ -1364,8 +1364,19 @@
 }
 
 func TestImportPath(t *testing.T) {
-	if path := TypeOf(&base64.Encoding{}).Elem().PkgPath(); path != "encoding/base64" {
-		t.Errorf(`TypeOf(&base64.Encoding{}).Elem().PkgPath() = %q, want "encoding/base64"`, path)
+	tests := []struct {
+		t    Type
+		path string
+	}{
+		{TypeOf(&base64.Encoding{}).Elem(), "encoding/base64"},
+		{TypeOf(uint(0)), ""},
+		{TypeOf(map[string]int{}), ""},
+		{TypeOf((*error)(nil)).Elem(), ""},
+	}
+	for _, test := range tests {
+		if path := test.t.PkgPath(); path != test.path {
+			t.Errorf("%v.PkgPath() = %q, want %q", test.t, path, test.path)
+		}
 	}
 }