constructor(cond, negate) {
    super();
    this.cond = cond;
    this.elses = [];
    if (negate && negate.nodeName) {
      this.block = negate;
    } else {
      this.negate = negate;
    }
  }
Initialize a new `If` with the given `cond`.

@param {Expression} cond
@param {Boolean|Block} negate, block
@api public