In a generator function, the parser rewrites a return statement into a "final"
yield.  A final yield used to close the generator, which was incorrect because
the return may occur inside a try-finally clause and so the generator may not
yet terminate.

BUG=

Review URL: https://codereview.chromium.org/1634553002

Cr-Commit-Position: refs/heads/master@{#33537}