gofmt: fix //line handling

Fixes #2369.

R=gri
CC=golang-dev
https://golang.org/cl/5275048

src/cmd/gofmt/gofmt_test.go

@@ -77,6 +77,7 @@ var tests = []struct {
 	{"testdata/rewrite1.input", "-r=Foo->Bar"},
 	{"testdata/rewrite2.input", "-r=int->bool"},
 	{"testdata/stdin*.input", "-stdin"},
+	{"testdata/comments.input", ""},
 }
 
 func TestRewrite(t *testing.T) {

src/cmd/gofmt/testdata/comments.golden

@@ -0,0 +1,9 @@
+package main
+
+func main() {}
+
+// comment here
+
+func f() {}
+
+//line foo.go:1

src/cmd/gofmt/testdata/comments.input

@@ -0,0 +1,9 @@
+package main
+
+func main() {}
+
+// comment here
+
+func f() {}
+
+//line foo.go:1

src/pkg/go/scanner/scanner.go

@@ -151,7 +151,7 @@ func (S *Scanner) interpretLineComment(text []byte) {
 					filename = filepath.Join(S.dir, filename)
 				}
 				// update scanner position
-				S.file.AddLineInfo(S.lineOffset, filename, line-1) // -1 since comment applies to next line
+				S.file.AddLineInfo(S.lineOffset+len(text)+1, filename, line) // +len(text)+1 since comment applies to next line
 			}
 		}
 	}