Remove dependency on Object.observe
Remove dependency on Object.observe

This makes getPolymerElements methods slower by a lot, as the caching is removed. But no longer requires O.o. This function is not known to be a critical path in tracing so it seems safe to be a bit slower in this instance.

BUG=catapult:#1848
TBR=fmeawad

Review URL: https://codereview.chromium.org/1526863002

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