go.tools/go/types: fix build for Go 1.1 users

Fixes golang/go#6485.

R=gri
CC=golang-dev
https://golang.org/cl/13901045

go/types/expr.go

@@ -1166,7 +1166,7 @@ func (check *checker) expr0(x *operand, e ast.Expr, hint Type) exprKind {
 		switch typ := x.typ.Underlying().(type) {
 		case *Basic:
 			if isString(typ) {
-				if e.Slice3 {
+				if slice3(e) {
 					check.invalidOp(x.pos(), "3-index slice of string")
 					goto Error
 				}
@@ -1211,14 +1211,14 @@ func (check *checker) expr0(x *operand, e ast.Expr, hint Type) exprKind {
 		}
 
 		// spec: "Only the first index may be omitted; it defaults to 0."
-		if e.Slice3 && (e.High == nil || e.Max == nil) {
+		if slice3(e) && (e.High == nil || sliceMax(e) == nil) {
 			check.errorf(e.Rbrack, "2nd and 3rd index required in 3-index slice")
 			goto Error
 		}
 
 		// check indices
 		var ind [3]int64
-		for i, expr := range []ast.Expr{e.Low, e.High, e.Max} {
+		for i, expr := range []ast.Expr{e.Low, e.High, sliceMax(e)} {
 			x := int64(-1)
 			switch {
 			case expr != nil:

go/types/go11.go

@@ -0,0 +1,17 @@
+// 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.
+
+// +build !go1.2
+
+package types
+
+import "go/ast"
+
+func slice3(x *ast.SliceExpr) bool {
+	return false
+}
+
+func sliceMax(x *ast.SliceExpr) ast.Expr {
+	return nil
+}

go/types/go12.go

@@ -0,0 +1,17 @@
+// 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.
+
+// +build go1.2
+
+package types
+
+import "go/ast"
+
+func slice3(x *ast.SliceExpr) bool {
+	return x.Slice3
+}
+
+func sliceMax(x *ast.SliceExpr) ast.Expr {
+	return x.Max
+}