Fixate target size in spark-splitter on startup

TBR: @jmesserly 

John, can you figure out why explicit propagation of a value of an already data-bound attribute was necessary in 2 places, and even that wasn't enough (on both platforms in one case and only in dart2js code in another case) and I had to explicitly call `propertyChanged()` (even `deliverChanges()` didn't work in dart2js code, although it did in Dartium)?