Mojom parser: Fix snippet algorithm.

mojom/mojom_parser/lexer/tokens.go

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 //
 // TokenKinds is a type which describes the kinds of tokens which can be
 // encountered in a mojom file.
 
 package lexer
 
 import (
@@ skipping 248 matching lines @@
                         begin += (index + 1)
                         break
                 }
         }
 
         // Now we calculate the end of the snippet line. Either the first new line
         // rune or a total of no more than 79 characters.
         end := len(source)
         for index, rune := range source[token.SourcePosBytes:] {
                 if rune == '\n' || runeCount >= 78 {
-                        end = index + begin + token.LinePosBytes
+                        end = index + token.SourcePosBytes
                         break
                 }
                 runeCount++
         }
 
         snippetBuffer := bytes.NewBufferString(source[begin:end])
         snippetBuffer.WriteRune('\n')
 
         // We calculate how much whitespace to add before the caret marker for the
         // token. Please note that there is an assumption that all non-tab characters
@@ skipping 38 matching lines @@
         }
 
         text := token.Text
         length := len(text)
         if (length < 2) || (text[0] != '"') || (text[length-1] != '"') {
                 panic(fmt.Sprintf("Lexer returned a string literal token whose "+
                         "text was not delimited by quotation marks: '%s'.", text))
         }
         return text[1 : length-1]
 }