Some fixes prompted by update to polymer 0.10.0-pre.9

@jmesserly TBRing this, as it is urgent and shouldn't be too contentious.

These PR should unblock the most urgent problems triggered by the update:
* An NPE in bindKeybindingDesc()
* Polymer errors along the lines of "spark.widgets.SomeWidget is not a subtype of spark.widgets.SomeWidget", which were related to #500, now seemingly fixed

The PR also:
* Removes a stray obsolete `lightdom` attribute on a Polymer element, which had to be removed long ago
* Once again waits for `Polymer.onReady()` before moving on with Spark startup (this one is a bit of shooting in the dark, but it should only improve the state of things, if anything).