Ensure fromAbove for downwards bindings.

[kaste]

Essentially downwards flowing data change events shouldn't echo upwards.

This is a breaking change since you actually had a test (!) that ensures the opposite.

[kaste]

Haha, I just saw #3077.

Note that the main problem here and #2815 was the new cached events systems, which was by far more surprising and breaking than this.

[kevinpschaaf]

Going to go ahead and close this due to the breaking change issue, as noted in the code and follow-up to #3077. As you saw we tried to do this and needed to revert it due to users calling addEventListener for prop-changed events on bound elements. That said, the plan is to break this in the next major release of Polymer (which will have a lot of this code refactored).