JDK 8 M6 addition: JEP 139: Enhance javac to Improve Build Speed

[mark.reinhold at oracle.com]

With the new build system (JEP 138 [1]) on its way it makes sense to
include the fast-javac work as well, so I've targeted JEP 139 [2] to
JDK 8 M6.

- Mark


[1] http://openjdk.java.net/jeps/138
[2] http://openjdk.java.net/jeps/139