ES6 shim cleanup

Use Map and Set instead of plain objects.

Using HEAD of hamony-collections while waiting for a release since,
https://github.com/Benvie/harmony-collections/pull/12

Also, could be useful but worked around for the moment,
https://github.com/Benvie/harmony-collections/pull/13

Bug: 53241
Change-Id: Id76b48f7f6f5fa53274512f73a45d5260fef795a
Merge "ES6 shim cleanup"