Standardize which properties CustomElement.observe() reports,
regardless of whether or not browser natively supports Object.observe().

Also may inadvertently fix a bug with CustomElement#_getProps()
returning a list with duplicate entries.

Refs ibm-js/decor#17, ibm-js/decor#22, and depends on
ibm-js/decor@ae66259f04bf7e665c6666ce8ff78c058837b4ff.