Suggested changes to #1205's weak value map semantics and implementation.

[erights]

My suggestions on #1205 's finalize.js were better expressed as a separate PR.

However, this suggestion as is breaks something. I'm not sure what. So be careful in acting on these suggestions.

Take the inline comments as commentary for you, not as comments to include.

We do not need to iterate on this PR. Take anything you think adds value, or merge it into your's if you'd like.