Minor tweaks to reuse objects and skip forEach during boot

Reuse the descriptor objects where possible. For non generational
GC this reduces the GC churn.

Use c-style for loops instead of forEach since forEach is slow
Make the window.onerror test more resilient to old browsers

Check the number of arguments passed to window.onerror before
asserting the value of the argument.
Make sure we use the wrapper NodeList in querySelector.js
Merge pull request #416 from arv/fix-window-onerror-test

Make the window.onerror test more resilient to old browsers
Merge pull request #415 from arv/random-minor-things

Minor tweaks to reuse objects and skip forEach during boot
Merge pull request #418 from arv/node-list

Make sure we use the wrapper NodeList in querySelector.js
Fix getElementsByTagName to handle colon and more

Instead of relying on CSS selectors, this now implements the
matching as described by dom.spec.whatwg.org

Fixes https://github.com/Polymer/polymer/issues/468