Test for memory leaks

[VladUreche]

When compiling multiple times:

• keep a weak reference to the existing RootContext
• after compiling, do System.gc()
• assert that the weak reference is dead, so the RootContext was gc-ed
• later refinement: use yourkit api to print out the links from the root (alive) objects to the objects that were supposed to be dead

[VladUreche]

Tentatively assigned to myself, but please feel free to take over the bug.

[dragos]

I think it's probably better to be more fuzzy about it. We have a memory leaks test in the IDE.

[VladUreche]

Yes, it would be good to also have a memory cap test (which should be fuzzy) -- I may be wrong but I think @axel22 has this already working in scalameter.

[DarkDimius]

I have implemented Weak-tracking of Context's during all DottyTests. I believe that it's a better idea than to write customs tests.
https://github.com/DarkDimius/dotty/tree/leaks
But this implementation requires an unreleased build of junit-interface.
Please also note that at least one context always persists as some objects(eg NoDenotation) capture last seen context.

[DarkDimius]

Merged and run with every test.
That's a hack that could show false positives, but I haven't seen them after half a year.