dom-bind-behavior bug against Polymer 1.9

[haobibo]

I‘m trying to resolve the dependency hell of bower packages in #560, but a bug in dom-bind-behavior leads to test fail.

Given that PolymerElements (e.g.: test-fixture) keeps updating, we need either

• a) peg elements version like test-fixture, so that PolymerElements will not drag polymer version (this seems not a long term solution because elements like declarativewidgets_explorer may depend on a higher version of polymer directly or indirectly), or
• b) keep maintaining urth-core-* elements, so that they work well with the latest version of Polymer (at least 1.x).

I'm trying to take the b) approach, and just found a bug in dom-bind-behavior when polymer upgrade from 1.5 to 1.9 in this build, in which the failed test is the urth-core-bind test suit: "should use proper channel data" (cascade of urth-core-bind element).

After a little bit dig, I found the bug is caused by this line of code: https://github.com/jupyter-widgets/declarativewidgets/blob/master/elements/urth-core-bind/dom-bind-behavior.html#L141

Works well with Polymer 1.5

Won't render in Polymer 1.9