We currently specify 'sourceCompatibility = 10' in build.gradle for building FX classes, which maps to '-source 1.10' and '-target 1.10'.

Once openjfx requires JDK 11 to build and run (see JDK-8209966), we should change this to build FX classes with ''sourceCompatibility = 11' , which maps to '-source 11' and '-target 11'.

This will allow us to use JDK 11 language features, and also will cause a fast-fail if a down-rev JDK is used.