Needed for intelligently aggregated coverage in the plugins.

cparty chat:
per-test coverage should be ignored where the source file that defined the test that caused the coverage since changed
then, we have covered all bases
only really needs a bit of extra data in the .s file to know when to discard it. will leave that to a post-2.5 JIRA roundup
so if someone is iteratively changing Foo.java and FooTest.java to get 100% coverage (or extrapolate to package level if you want), saving FooTest.java will invalidate the coverage of Foo.java that was caused by the old version of FooTest.java
in that to make CEP truely useable and accurate, that's what we need