json: calculate Offset for Indent correctly

Fixes #2171

This is the real change.

R=adg
CC=golang-dev, r, rsc
https://golang.org/cl/4943041

src/pkg/json/indent.go

@@ -59,6 +59,7 @@ func Indent(dst *bytes.Buffer, src []byte, prefix, indent string) os.Error {
 	needIndent := false
 	depth := 0
 	for _, c := range src {
+		scan.bytes++
 		v := scan.step(&scan, int(c))
 		if v == scanSkipSpace {
 			continue

src/pkg/json/scanner_test.go

@@ -7,7 +7,9 @@ package json
 import (
 	"bytes"
 	"math"
+	"os"
 	"rand"
+	"reflect"
 	"testing"
 )
 
@@ -136,6 +138,29 @@ func TestIndentBig(t *testing.T) {
 	}
 }
 
+type indentErrorTest struct {
+	in  string
+	err os.Error
+}
+
+var indentErrorTests = []indentErrorTest{
+	{`{"X": "foo", "Y"}`, &SyntaxError{"invalid character '}' after object key", 17}},
+	{`{"X": "foo" "Y": "bar"}`, &SyntaxError{"invalid character '\"' after object key:value pair", 13}},
+}
+
+func TestIdentErrors(t *testing.T) {
+	for i, tt := range indentErrorTests {
+		slice := make([]uint8, 0)
+		buf := bytes.NewBuffer(slice)
+		if err := Indent(buf, []uint8(tt.in), "", ""); err != nil {
+			if !reflect.DeepEqual(err, tt.err) {
+				t.Errorf("#%d: Indent: %#v", i, err)
+				continue
+			}
+		}
+	}
+}
+
 func TestNextValueBig(t *testing.T) {
 	initBig()
 	var scan scanner