anchor.diff:      // in a reference to the "of" keyword.
       var ofKeyword = partKeyword.next;
       List<Annotation> metadata = pop();
  -    Comment comment = null; // TODO(paulberry)
  +    // TODO(paulberry): capture doc comments.  See dartbug.com/28851.
  +    Comment comment = null;
       push(ast.partOfDirective(comment, metadata, toAnalyzerToken(partKeyword),
           toAnalyzerToken(ofKeyword), uri, name, toAnalyzerToken(semicolon)));
       accumulateIdentifierComponents = false;
  
positive.code:     StringLiteral uri = pop();
      List<Annotation> metadata = pop();
      assert(metadata == null);
      // TODO(paulberry): capture doc comments.
      Comment comment = null;
      push(ast.exportDirective(comment, metadata, toAnalyzerToken(exportKeyword),
          uri, configurations, combinators, toAnalyzerToken(semicolon)));
  
negative.code:     List<SimpleIdentifier> libraryName = pop();
      var name = ast.libraryIdentifier(libraryName);
      List<Annotation> metadata = pop();
      // TODO(paulberry): capture doc comments.  See dartbug.com/28851.
      Comment comment = null;
      push(ast.libraryDirective(comment, metadata,
          toAnalyzerToken(libraryKeyword), name, toAnalyzerToken(semicolon)));
      accumulateIdentifierComponents = false;
  