ExecutorService's type parameters changed between JDK5 and JDK6 in a
way that makes it impossible for our invokeAll/invokeAny methods to
match both at compile time. This project builds a JDK6-like copy of
ExecutorService (but with JDK5 compiler settings to ensure that it will
work with JRE5 at runtime). This project's is then used in the bootstrap
class path of Guava proper.