move comment tokens into preceedingComments field

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 31 matching lines @@
    * is not exposed to clients of the scanner, which are expected to invoke
    * [firstToken] to access the token stream.
    */
   final Token tokens = new SymbolToken(EOF_INFO, -1);
 
   /**
    * A pointer to the last scanned token.
    */
   Token tail;
 
+  /**
+   * A pointer to the stream of comment tokens created by this scanner
+   * before they are assigned to the [Token] precedingComments field
+   * of a non-comment token. A value of `null` indicates no comment tokens.
+   */
+  Token comments;
+
+  /**
+   * A pointer to the last scanned comment token or `null` if none.
+   */
+  Token commentsTail;
+
   final List<int> lineStarts;
 
   AbstractScanner(this.includeComments, {int numberOfBytesHint})
       : lineStarts = new LineStarts(numberOfBytesHint) {
     this.tail = this.tokens;
   }
 
   /**
    * Advances and returns the next character.
    *
@@ skipping 1177 matching lines @@
       switchToUint32(newLength);
     }
   }
 
   void switchToUint32(int newLength) {
     final newArray = new Uint32List(newLength);
     newArray.setRange(0, arrayLength, array);
     array = newArray;
   }
 }