cmd/gc: tag builtin error, byte, rune to avoid exporting them.

Fixes #5910.
Fixes #6260.

R=golang-dev, daniel.morsing
CC=golang-dev
https://golang.org/cl/13257044

src/cmd/gc/lex.c

@@ -2008,27 +2008,27 @@ lexinit1(void)
 	// error type
 	s = lookup("error");
 	s->lexical = LNAME;
-	errortype = t;
-	errortype->sym = s;
 	s1 = pkglookup("error", builtinpkg);
+	errortype = t;
+	errortype->sym = s1;
 	s1->lexical = LNAME;
 	s1->def = typenod(errortype);
 
 	// byte alias
 	s = lookup("byte");
 	s->lexical = LNAME;
-	bytetype = typ(TUINT8);
-	bytetype->sym = s;
 	s1 = pkglookup("byte", builtinpkg);
+	bytetype = typ(TUINT8);
+	bytetype->sym = s1;
 	s1->lexical = LNAME;
 	s1->def = typenod(bytetype);
 
 	// rune alias
 	s = lookup("rune");
 	s->lexical = LNAME;
-	runetype = typ(TINT32);
-	runetype->sym = s;
 	s1 = pkglookup("rune", builtinpkg);
+	runetype = typ(TINT32);
+	runetype->sym = s1;
 	s1->lexical = LNAME;
 	s1->def = typenod(runetype);
 }

test/fixedbugs/bug460.dir/a.go

@@ -6,4 +6,8 @@ package a
 
 type Foo struct {
 	int
+	int8
+	error
+	rune
+	byte
 }

test/fixedbugs/bug460.dir/b.go

@@ -9,6 +9,9 @@ import "./a"
 var x a.Foo
 
 func main() {
-	x.int = 20 // ERROR "unexported field"
+	x.int = 20    // ERROR "unexported field"
+	x.int8 = 20   // ERROR "unexported field"
+	x.error = nil // ERROR "unexported field"
+	x.rune = 'a'  // ERROR "unexported field"
+	x.byte = 20   // ERROR "unexported field"
 }
-

test/fixedbugs/issue5910.dir/a.go

+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
 package a
 
 type Package struct {
@@ -7,11 +11,12 @@ type Package struct {
 type Future struct {
 	result chan struct {
 		*Package
+		error
 	}
 }
 
-func (t *Future) Result() *Package {
+func (t *Future) Result() (*Package, error) {
 	result := <-t.result
 	t.result <- result
-	return result.Package
+	return result.Package, result.error
 }

test/fixedbugs/issue5910.dir/main.go

+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
 package main
 
 import "a"