I feel uncomfortable with this asymmetry: target size is set from the preferences in one class, but updated in another. One way (the easiest) to restore the symmetry would be to add `SparkPolymer.onSplitViewUpdate(int newPosition)` and call it from `SparkPolymerUI.onSplitterUpdate()`. Another way (probably more Polymer-y, as it's more aligned with the life cycle of Polymer elements) would be to do this initialization in `SparkPolymerUI.created()`.