Declaring a scripted class pins its interpreter #843

After evaluating class Foo { } in an Interpreter (whether or not it's ever instantiated), dropping the interpreter and running System.gc() up to 50 times still leaves it reachable via a WeakReference. A script with no class declaration (x = 1;) is collected normally under the same driver. This looks like the same underlying cause as the class-manager pinning issues filed separately, though it was measured with a different driver (this one doesn't first move the static loader on to another interpreter).