join(aSep) {
    let newChildren
    let i
    const len = this.children.length
    if (len > 0) {
      newChildren = []
      for (i = 0; i < len - 1; i++) {
        newChildren.push(this.children[i])
        newChildren.push(aSep)
      }
      newChildren.push(this.children[i])
      this.children = newChildren
    }
    return this
  }
Like `String.prototype.join` except for SourceNodes. Inserts `aStr` between
each of `this.children`.

@param aSep The separator.