In make/langtools/src/classes/build/tools/symbolgenerator/TransitiveDependencies.java, javac gets called with a fixed command line. This includes the arguments "-source 9 -target 9". While this was fine for jdk9, in jdk10 this results in the following warning:

warning: [options] bootstrap class path not set in conjunction with -source 1.9

I presume it is a mistake that this was not updated to -source 10 -target 10 for jdk10.