constructor(val, key, expr, block) {
    super();
    this.val = val;
    this.key = key;
    this.expr = expr;
    this.block = block;
  }
Initialize a new `Each` node with the given `val` name,
`key` name, `expr`, and `block`.

@param {String} val
@param {String} key
@param {Expression} expr
@param {Block} block
@api public