isNumeric_(c, decimal) {
    decimal = decimal !== undefined ? decimal : false;
    return (c >= '0' && c <= '9') || (c == '.' && !decimal);
  }
@param {string} c Character.
@param {boolean} [decimal] Whether the string number
    contains a dot, i.e. is a decimal number.
@return {boolean} Whether the character is numeric.
@private