Ycheck failures after frontend

[DarkDimius]

Some tests fail Ycheck after frontend

./tests/pos/java-interop/t2413/TestScalac.scala
./tests/pos/jon.scala
./tests/run/Course-2002-01.scala

[smarter]

All of the tests mentioned now pass Ycheck:frontend, but we should check if others don't and if we can reenable Ycheck:all.

[allanrenucci]

I recently tried to Ycheck:all and we still have failure after frontend for:

• pos/t1513a.scala
• pos/t1513b.scala
• pos/reference/named-typeargs.scala
• tests/run/t7126.scala
• tests/run/hmap.scala
• tests/run/hmap-covariant.scala
• compiler/src/dotty/tools/dotc/core/Hashable.scala (see CheckInlineConformant is too strict for -Ycheck:frontend #3971)
• compiler/src/dotty/tools/dotc/transform/FunctionalInterfaces.scala
• collection-strawman/collections/src/main/scala/strawman/collection/BitSet

[allanrenucci]

Closed in #4183