CPS Transformer: Let sent throw as needed

This allows us to reduce the amount of code we generate.

Fixes #875

There are three cases:
1. yield expr: In this case we need to insert `$ctx.maybeThrow`
2. x = yield expr: In this case the $ctx.sent getter handles the possible throwing
3. yield* expr: In this case the throwing is handled in the inner iterator.

Review URL: https://github.com/google/traceur-compiler/pull/880

Closes #880, closes #875.