code: private Token eat(TokenType expectedTokenType) {
      Token token = nextToken();
      if (token.type != expectedTokenType) {
        reportExpectedError(token, expectedTokenType);
        return null;
      }
      return token;
    }
docs: Consumes the next token. If the consumed token is not of the expected type then
  report an error and return null. Otherwise return the consumed token.
  
  @param expectedTokenType
  @return The consumed token, or null if the next token is not of the expected type.