spark-split-view: Correct styling.

The reason about  https://github.com/dart-lang/spark/issues/1086  is the same with https://github.com/dart-lang/spark/issues/733

fixes https://github.com/dart-lang/spark/issues/1086
review @ussuri

*TEST* = 1. `widgets/example/spark_splitter/ & spark_split_view/` with  Dartium M31, M33.
                2. Run Spark with Dartium M31, M33.
                3. Run deployed(./grind deploy) Spark with Chrome beta M33.
                 (cannot Tested with M32, M31. for M32 spark doesn't work without this PR)