This is an umbrella task for writing tests that verify invariants in JavaFX.
See discussion in https://github.com/openjdk/jfx/pull/1485 for the origin of this task.
Also https://bugs.openjdk.org/browse/JDK-8341010

Specifically:
- properties return the right object in their getBean() method (AreaChart:109, StackedAreaChart:109)
- property setter: setXXX(valueA) and getXXXProperty().set(valueA) should return the same value (but not necessarily == value) via getXXX() and getXXXProperty().get().
- property setXXX(v) and getXXXProperty().set(v) should be equivalent (with exceptions, see Labeled:300 setEllipsisString())