Cannot bind elements by @Id when template extend Polymer.mixinBehavior

[oluwasayo]

Since Flow Alpha13, binding to elements by @Id when the template extends Polymer.mixinBehavior([Polymer.IronResizableBehavior], Polymer.Element) results in a "cannot read property my-id of undefined" in the browser console.

To reproduce:
Alpha12 works: https://github.com/oluwasayo/skeleton-starter-flow/tree/flow-alpha12-works
Alpha16 broken: https://github.com/oluwasayo/skeleton-starter-flow/tree/flow-alpha16-broken

Regression from: #3098

[manolo]

Related with #3264

[denis-anisimov]

The example in the ticket description is technically invalid.
It uses Polymer.IronResizableBehavior mixin which is not imported by anything in the example and as a result ht behavior is not available (which can be seen in the browser console as a warning).
Nevertheless there is a bug in the Flow.
For some reasons when mixin is used the element may have root property but doesn't have $ property. And that causes the issue.
Even though the example is incorrect it still should work (ignoring non-existing) behavior.
One may just remove Polymer.IronResizableBehavior from the mixin declaration (and keep mixins array empty) and the issue still will be there though now the example is absolutely correct.

Resolution: we should check $ when it's needed, not only root property.