Add Throwable and Error; modify Exception so it's a subclass of Throwable (see ceylon/ceylon-compiler#1522)
Change Closeable's exception parameter to be a Throwable (see ceylon/ceylon-compiler#1522)
Beef-up the exceptions tests a little bit (see ceylon/ceylon-compiler#1522)
Improve metamodel tests around throwables (see ceylon/ceylon-compiler#1522)
Fix a bug where we weren't computing the reified type of a sequential properly.