diff --git a/src/pkg/go/token/position.go b/src/pkg/go/token/position.go
index 0044a0ed77d..809e53f0aa2 100644
--- a/src/pkg/go/token/position.go
+++ b/src/pkg/go/token/position.go
@@ -153,7 +153,7 @@ type lineInfo struct {
 
 // AddLineInfo adds alternative file and line number information for
 // a given file offset. The offset must be larger than the offset for
-// the previously added alternative line info and not larger than the
+// the previously added alternative line info and smaller than the
 // file size; otherwise the information is ignored.
 //
 // AddLineInfo is typically used to register alternative position
@@ -161,7 +161,7 @@ type lineInfo struct {
 //
 func (f *File) AddLineInfo(offset int, filename string, line int) {
 	f.set.mutex.Lock()
-	if i := len(f.infos); i == 0 || f.infos[i-1].offset < offset && offset <= f.size {
+	if i := len(f.infos); i == 0 || f.infos[i-1].offset < offset && offset < f.size {
 		f.infos = append(f.infos, lineInfo{offset, filename, line})
 	}
 	f.set.mutex.Unlock()
@@ -212,27 +212,30 @@ func (f *File) LineCount() int {
 
 // AddLine adds the line offset for a new line.
 // The line offset must be larger than the offset for the previous line
-// and not larger than the file size; otherwise the line offset is ignored.
+// and smaller than the file size; otherwise the line offset is ignored.
 //
 func (f *File) AddLine(offset int) {
 	f.set.mutex.Lock()
-	if i := len(f.lines); (i == 0 || f.lines[i-1] < offset) && offset <= f.size {
+	if i := len(f.lines); (i == 0 || f.lines[i-1] < offset) && offset < f.size {
 		f.lines = append(f.lines, offset)
 	}
 	f.set.mutex.Unlock()
 }
 
 
-// SetLines sets all line offsets for a file and returns true if successful.
+// SetLines sets the line offsets for a file and returns true if successful.
+// The line offsets are the offsets of the first character of each line;
+// for instance for the content "ab\nc\n" the line offsets are {0, 3}.
+// An empty file has an empty line offset table.
 // Each line offset must be larger than the offset for the previous line
-// and not larger than the file size; otherwise the SetLines fails and returns
+// and smaller than the file size; otherwise SetLines fails and returns
 // false.
 //
 func (f *File) SetLines(lines []int) bool {
 	// verify validity of lines table
 	size := f.size
 	for i, offset := range lines {
-		if i > 0 && offset <= lines[i-1] || size < offset {
+		if i > 0 && offset <= lines[i-1] || size <= offset {
 			return false
 		}
 	}
@@ -245,6 +248,27 @@ func (f *File) SetLines(lines []int) bool {
 }
 
 
+// SetLinesForContent sets the line offsets for the given file content.
+func (f *File) SetLinesForContent(content []byte) {
+	var lines []int
+	line := 0
+	for offset, b := range content {
+		if line >= 0 {
+			lines = append(lines, line)
+		}
+		line = -1
+		if b == '\n' {
+			line = offset + 1
+		}
+	}
+
+	// set lines table
+	f.set.mutex.Lock()
+	f.lines = lines
+	f.set.mutex.Unlock()
+}
+
+
 // Pos returns the Pos value for the given file offset;
 // the offset must be <= f.Size().
 // f.Pos(f.Offset(p)) == p.
diff --git a/src/pkg/go/token/position_test.go b/src/pkg/go/token/position_test.go
index 1cffcc3c278..979c9b1e8e7 100644
--- a/src/pkg/go/token/position_test.go
+++ b/src/pkg/go/token/position_test.go
@@ -39,14 +39,18 @@ func TestNoPos(t *testing.T) {
 
 var tests = []struct {
 	filename string
+	source   []byte // may be nil
 	size     int
 	lines    []int
 }{
-	{"a", 0, []int{}},
-	{"b", 5, []int{0}},
-	{"c", 10, []int{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}},
-	{"d", 100, []int{0, 5, 10, 20, 30, 70, 71, 72, 80, 85, 90, 99}},
-	{"e", 777, []int{0, 80, 100, 120, 130, 180, 267, 455, 500, 567, 620}},
+	{"a", []byte{}, 0, []int{}},
+	{"b", []byte("01234"), 5, []int{0}},
+	{"c", []byte("\n\n\n\n\n\n\n\n\n"), 9, []int{0, 1, 2, 3, 4, 5, 6, 7, 8}},
+	{"d", nil, 100, []int{0, 5, 10, 20, 30, 70, 71, 72, 80, 85, 90, 99}},
+	{"e", nil, 777, []int{0, 80, 100, 120, 130, 180, 267, 455, 500, 567, 620}},
+	{"f", []byte("package p\n\nimport \"fmt\""), 23, []int{0, 10, 11}},
+	{"g", []byte("package p\n\nimport \"fmt\"\n"), 24, []int{0, 10, 11}},
+	{"h", []byte("package p\n\nimport \"fmt\"\n "), 25, []int{0, 10, 11, 24}},
 }
 
 
@@ -77,10 +81,26 @@ func verifyPositions(t *testing.T, fset *FileSet, f *File, lines []int) {
 }
 
 
+func makeTestSource(size int, lines []int) []byte {
+	src := make([]byte, size)
+	for _, offs := range lines {
+		if offs > 0 {
+			src[offs-1] = '\n'
+		}
+	}
+	return src
+}
+
+
 func TestPositions(t *testing.T) {
 	const delta = 7 // a non-zero base offset increment
 	fset := NewFileSet()
 	for _, test := range tests {
+		// verify consistency of test case
+		if test.source != nil && len(test.source) != test.size {
+			t.Errorf("%s: inconsistent test case: expected file size %d; got %d", test.filename, test.size, len(test.source))
+		}
+
 		// add file and verify name and size
 		f := fset.AddFile(test.filename, fset.Base()+delta, test.size)
 		if f.Name() != test.filename {
@@ -107,15 +127,26 @@ func TestPositions(t *testing.T) {
 			verifyPositions(t, fset, f, test.lines[0:i+1])
 		}
 
-		// add lines at once and verify all positions
-		ok := f.SetLines(test.lines)
-		if !ok {
+		// add lines with SetLines and verify all positions
+		if ok := f.SetLines(test.lines); !ok {
 			t.Errorf("%s: SetLines failed", f.Name())
 		}
 		if f.LineCount() != len(test.lines) {
 			t.Errorf("%s, SetLines: expected line count %d; got %d", f.Name(), len(test.lines), f.LineCount())
 		}
 		verifyPositions(t, fset, f, test.lines)
+
+		// add lines with SetLinesForContent and verify all positions
+		src := test.source
+		if src == nil {
+			// no test source available - create one from scratch
+			src = makeTestSource(test.size, test.lines)
+		}
+		f.SetLinesForContent(src)
+		if f.LineCount() != len(test.lines) {
+			t.Errorf("%s, SetLinesForContent: expected line count %d; got %d", f.Name(), len(test.lines), f.LineCount())
+		}
+		verifyPositions(t, fset, f, test.lines)
 	}
 }