improve on-* delegation by introducing control for Polymer-bubbling (as distinct from DOM bubbling)

[sjmiles]

For example, polymer-signals has a problem because the signal-notifier events need to be non-bubbling, but non-bubbling events foil the on-* delegation system.

We can have our cake and eat it too if the events bubble in DOM but not during Polymer delegation.

[sjmiles]

This issue is old and the concept of polymer delegation is currently deceased (although there is periodic talk of reincarnation). Closing.