compiler.codecache.stress.Helper.TestCaseImpl is a nested inner class of
compiler.codecache.stress.Helper so they have nestmate relationship.
However the test defines TestCaseImpl class with a different class loader in
a different runtime package as its nest host class
compiler.codecache.stress.Helper.

TestCaseImpl has a NestHost attribute but TestCaseImpl does not rely on
nestmate access it never validates the nest relationship and so this
problem is not seen.

These tests must be fixed before JDK-8171335 that will change the lambda
generation using dynamic nestmates where the nest-host of the nested class
has to be retrieved and results in a failure:

Exception: java.lang.IncompatibleClassChangeError: Type compiler.codecache.stress.Helper$TestCaseImpl (loader: jdk.test.lib.ByteCodeLoader ...) is not a nest member of compiler.codecache.stress.Helper (loader: 'app'): types are in different packages

The nest-host and the nest members must all be in the same package.
The test should be rewritten to use top-level classes rather then nested ones.

http://hg.openjdk.java.net/valhalla/valhalla/rev/ab7ea72963c9
is the patch fixing these tests in valhalla repo nestmates branch.