Checklist
- New JDK reaches unstable.
java_defaults.mk in java-common updated to be aware of the new JVM version.
- Test rebuilds and mass bug filing.
- Most bugs fixed.
Transition java-common.
Common problems
JDK upgrade having dropped older source versions, see Source#MIN.
maven-javadoc-plugin failing to find the javadoc tool, as it relies on JAVA_HOME which we try not to set. e.g. fix for Java9.