make String work on Position values, to enable

	fmt.Printf("%s: %s\n", expr.Pos(), message);

R=gri
DELTA=15  (1 added, 3 deleted, 11 changed)
OCL=34706
CL=34708

src/pkg/go/token/token.go

@@ -352,8 +352,7 @@ func (pos *Position) IsValid() bool {
 }
 
 
-func (pos *Position) String() string {
-	if pos != nil {
+func (pos Position) String() string {
 	s := pos.Filename;
 	if pos.IsValid() {
 		if s != "" {
@@ -361,9 +360,8 @@ func (pos *Position) String() string {
 		}
 		s += fmt.Sprintf("%d:%d", pos.Line, pos.Column);
 	}
-		if s != "" {
-			return s;
+	if s == "" {
+		s = "???";
 	}
-	}
-	return "<unknown position>";
+	return s;
 }