This CL's purpose is to introduce the dependency on the HTML Custom
Elements polyfill. Like we've done so far, I'm trying to keep
dependencies light by using current or polyfilling future webcomponents
standards.
Change-Id: I11d14db367b697cdd527fb66b9d7d160ac244b78
Reviewed-on: https://go-review.googlesource.com/25494
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
