Sharing backend between scalac and dotty.

Due to a lot of error and workarounds required for backend not regress in terms of speed, I guess the original idea was to high to be reached..
Nevertheless it was good to try.
Erasure is on by default.

Remove historical enabling of erasure for some tests: its enabled by default now
Update to scala 2.11.4
Hack ClassFileParser to not treat Null$ or Nothing$ as modules.

They are classes.
Move methods shared between Mixin and Backend to SymUtils.
Remove duplicate isAnonymousFunction
New phase: collectEntryPoints

collects entry points and registers them in backend
Depend on custom compiler with https://github.com/scala/scala/pull/4136 incorporated
remove dependency on Btypes from DottyPrimitives. Rely on symbols instead.
GenBcode phase implemented.

That doesn't mean it works :-)
Add member lookups needed by backend.
Complete DottyBackendInterface.
Enable new phases.
Disable backend by default in tests.
Mixing now also changes Scala2 traits into interfaces.
Modify runner script to include modified scalac on class path.
Add test for backend.
Update class-path dependant tests. Disable one.
Label test with nested cycles and simple patterns.
Implemented handling of <label> DefDefs in backend.