fix fasta scanner comment processing

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

 // Copyright (c) 2012, 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.abstract_scanner;
 
 import 'dart:collection' show ListMixin;
 
 import 'dart:typed_data' show Uint16List, Uint32List;
 
@@ skipping 879 matching lines @@
   void appendToken(Token token) {
     tail.next = token;
     tail.next.previous = tail;
     tail = tail.next;
     if (comments != null && comments == token.precedingComments) {
       comments = null;
       commentsTail = null;
     } else {
       // It is the responsibility of the caller to construct the token
       // being appended with preceeding comments if any
-      assert(comments == null || token.isSynthetic);
+      assert(comments == null || token.isSynthetic || token is ErrorToken);
     }
   }
 
   void _appendToCommentStream(Token newComment) {
     if (comments == null) {
       comments = newComment;
       commentsTail = comments;
     } else {
       commentsTail.next = newComment;
       commentsTail.next.previous = commentsTail;
@@ skipping 404 matching lines @@
       switchToUint32(newLength);
     }
   }
 
   void switchToUint32(int newLength) {
     final newArray = new Uint32List(newLength);
     newArray.setRange(0, arrayLength, array);
     array = newArray;
   }
 }