Polymer/ShadowDOM — pull request #239 by arv (2013-09-05 14:19:07+00:00)
Title: Use an expando property for the wrapper.

I did some simple benchmarks by traversing the HTML spec and a few times and this reduced the self time from 450ms to 25ms (30% to 3%). These numbers are not very exact but it is clear that this is a huge win.