Fix scanning of unterminated strings

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

 // Copyright (c) 2011, 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 dart2js.scanner.string_scanner;
 
 import '../../scanner/token.dart' show SyntheticStringToken, TokenType;
 
 import '../../scanner/token.dart' as analyzer show StringToken;
 
@@ skipping 42 matching lines @@
     return new StringToken.fromSubstring(
         type, string, start, scanOffset + extraOffset, tokenStart,
         canonicalize: true, precedingComments: comments);
   }
 
   @override
   analyzer.StringToken createSyntheticSubstringToken(
       TokenType type, int start, bool asciiOnly, String closingQuotes) {
     String source = string.substring(start, scanOffset);
     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.fromSubstring(
         type, string, start, scanOffset + extraOffset, tokenStart,
         canonicalize: true);
   }
 
@@ skipping 17 matching lines @@
 
   SubStringScanner(this.baseOffset, String string,
       {bool includeComments: false})
       : super(string, includeComments: includeComments);
 
   @override
   void beginToken() {
     tokenStart = baseOffset + stringOffset;
   }
 }