Fix scanning of unterminated strings

pkg/front_end/lib/src/fasta/scanner/utf8_bytes_scanner.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library fasta.scanner.utf8_bytes_scanner;
 
 import 'dart:convert' show UNICODE_BOM_CHARACTER_RUNE, UTF8;
 
 import '../../scanner/token.dart' show SyntheticStringToken, TokenType;
 
@@ skipping 195 matching lines @@
     return new StringToken.fromUtf8Bytes(
         type, bytes, start, byteOffset + extraOffset, asciiOnly, tokenStart,
         precedingComments: comments);
   }
 
   @override
   analyzer.StringToken createSyntheticSubstringToken(
       TokenType type, int start, bool asciiOnly, String closingQuotes) {
     String source = StringToken.decodeUtf8(bytes, start, byteOffset, asciiOnly);
     return new SyntheticStringToken(
-        type, source + closingQuotes, start, source.length);
+        type, source + closingQuotes, tokenStart, source.length);
   }
 
   @override
   CommentToken createCommentToken(TokenType type, int start, bool asciiOnly,
       [int extraOffset = 0]) {
     return new CommentToken.fromUtf8Bytes(
         type, bytes, start, byteOffset + extraOffset, asciiOnly, tokenStart);
   }
 
   @override
   DartDocToken createDartDocToken(TokenType type, int start, bool asciiOnly,
       [int extraOffset = 0]) {
     return new DartDocToken.fromUtf8Bytes(
         type, bytes, start, byteOffset + extraOffset, asciiOnly, tokenStart);
   }
 
   bool atEndOfFile() => byteOffset >= bytes.length - 1;
 }