improve fasta unterminated string recovery

pkg/front_end/lib/src/scanner/token.dart

 // Copyright (c) 2016, 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.
 
 /**
  * Defines the tokens that are produced by the scanner, used by the parser, and
  * referenced from the [AST structure](ast.dart).
  */
 import 'dart:collection';
 
@@ skipping 698 matching lines @@
   int get length => 0;
 
   @override
   Token copy() => new SyntheticKeywordToken(keyword, offset);
 }
 
 /**
  * A token whose value is independent of it's type.
  */
 class SyntheticStringToken extends StringToken {
+  final int _length;
+
   /**
    * Initialize a newly created token to represent a token of the given [type]
-   * with the given [value] at the given [offset].
+   * with the given [value] at the given [offset]. If the [length] is
+   * not specified, then it defaults to the length of [value].
    */
-  SyntheticStringToken(TokenType type, String value, int offset)
+  SyntheticStringToken(TokenType type, String value, int offset, [this._length])
       : super(type, value, offset);
 
   @override
   bool get isSynthetic => true;
 
   @override
+  int get length => _length ?? super.length;
+
+  @override
   Token copy() => new SyntheticStringToken(type, _value, offset);
 }
 
 /**
  * A synthetic token.
  */
 class SyntheticToken extends SimpleToken {
   SyntheticToken(TokenType type, int offset) : super(type, offset);
 
   @override
@@ skipping 940 matching lines @@
 
   void set precedingComments(CommentToken comment) {
     _precedingComment = comment;
     _setCommentParent(_precedingComment);
   }
 
   @override
   Token copy() =>
       new TokenWithComment(type, offset, copyComments(precedingComments));
 }