code: private ImmutableList<ParseTree> parseExportSpecifierSet() {
      ImmutableList.Builder<ParseTree> elements;
      elements = ImmutableList.builder();
      eat(TokenType.OPEN_CURLY);
      while (peekIdOrKeyword()) {
        elements.add(parseExportSpecifier());
        if (!peek(TokenType.CLOSE_CURLY)) {
          eat(TokenType.COMMA);
        }
      }
      eat(TokenType.CLOSE_CURLY);
      return elements.build();
    }
docs: ExportSpecifierSet ::= '{' (ExportSpecifier (',' ExportSpecifier)* (,)? )?  '}'