CPS Transformer: Clean up conditional state and blocks

Fixes #886 #887

Review URL: https://github.com/google/traceur-compiler/pull/888

Closes #888, closes #886.
CPS Transformer: Allow yield and return in finally

We disallowed yield in finally based on an old design. We have covered this in TC39 and it should be allowed.

Fortunately it just worked.

Review URL: https://github.com/google/traceur-compiler/pull/889

Closes #889.
traceur@0.0.31