function expectKeyword(lexer, value) {
  var token = lexer.token;
  if (token.kind === _lexer.TokenKind.NAME && token.value === value) {
    lexer.advance();
    return token;
  }
  throw (0, _error.syntaxError)(lexer.source, token.start, 'Expected "' + value + '", found ' + (0, _lexer.getTokenDesc)(token));
}
If the next token is a keyword with the given value, return that token after
advancing the lexer. Otherwise, do not change the parser state and return
false.