improve fasta unterminated string recovery

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 TokenType;
+import '../../scanner/token.dart' show SyntheticStringToken, TokenType;
+
+import '../../scanner/token.dart' as analyzer show StringToken;
 
 import '../scanner.dart' show unicodeReplacementCharacter;
 
 import 'token.dart' show CommentToken, DartDocToken, StringToken;
 
 import 'array_based_scanner.dart' show ArrayBasedScanner;
 
 /**
  * Scanner that reads from a UTF-8 encoded list of bytes and creates tokens
  * that points to substrings.
@@ skipping 171 matching lines @@
 
   int get stringOffset {
     if (stringOffsetSlackOffset == byteOffset) {
       return byteOffset - utf8Slack - 1;
     } else {
       return byteOffset - utf8Slack;
     }
   }
 
   @override
-  StringToken createSubstringToken(TokenType type, int start, bool asciiOnly,
+  analyzer.StringToken createSubstringToken(
+      TokenType type, int start, bool asciiOnly,
       [int extraOffset = 0]) {
     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);
+  }
+
+  @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;
 }