RFR (XS) 8188828 Intermittent ClassNotFoundException: jdk.test.lib.Platform for compiler tests

[Ioi Lam]

Please review this very simple change:

https://bugs.openjdk.java.net/browse/JDK-8188828
http://ioilinux.us.oracle.com/webrev/jdk10/8188828_compiler_test_class_not_found.v01/

The dependency of

     FileInstaller -> Utils -> JDKToolLauncher -> Platform

has caused many intermittent ClassNotFoundException in the hotspot 
nightly runs.
While this fix does not address the root cause (proper dependencies are not
specified in the test cases -- which we are planning to fix), we will 
hopefully
see much fewer occurrences of this annoying failure scenario.

Thanks a lot to Igor for suggesting the simple fix!

- Ioi