module/bootclass troubles with jdk9

[Martin Buchholz]

I'm trying to use jdk9-b34 and see that rt.jar and friends are gone.
Are we in jigsaw-land yet?
javac doesn't seem to know about any -modulepath flag yet.
(javac -help, javac -X is module-unaware)
How do I specify a bootclasspath for jdk9-b34?
You can't use a jimage with -bootclasspath, and -modulepath doesn't seem to
work (even though I see references to it in the javac sources)

How will one be able to use javac/jdk10 to target jdk9?  Traditionally,
-target 9 -source 9 and -bootclasspath with some collection of jdk9 jar
files.  But jdk9 bootclasspath jars are gone, and I don't see anything to
replace it?

(Background: trying to fix up jsr166 ant build script for jdk9-latest)