This has been resolved in jdk9/dev via JDK-7133499.
Implementing asynchronous close of FileChannel is going to require changes that may be disruptive to other platforms so I think we should work on this for 8 and then back-port later to a 7u release. Note that this never worked with Apple's JDK6 either so this is not a regression.
The issue here is that dup2 is blocking and doesn't close the file as is does on other platforms. This is the same underlying reason that channels/AsyncCloseAndInterrupt.java fails.