3 incompatibilities with scalac

[olafurpg]

See: https://github.com/olafurpg/dotty/pull/2/files

All three examples compile fine with scalac while dotc emits a compiler error. I don't know if these incompatibilities are intentional or known bugs.

[odersky]

Hi Olaf, can you make a PR against dotty master and put the tests in pos/pending please?

[olafurpg]

Done, see #1464.