function DomSequence(source) {
    this.source = source;
  }
A seqence of DOM nodes.

You can get a `DomSequence` by wrapping a `NodeList` or `HTMLCollection`
with `Lazy`:

    var paragraphs = Lazy(document.querySelectorAll('p'));

@public
@constructor
@param {NodeList|HTMLCollection} source The underlying collection of DOM
    nodes.