When calling the return method on a generator suspended inside a yield*, yield*
in turn calls return on its iterable.  If this results in a "done" iterator,
yield* must return immediately, thus terminating the generator.  For some
reason, we didn't terminate the generator but continued right after the yield*.

R=adamk@chromium.org
BUG=v8:5131

Review-Url: https://codereview.chromium.org/2100093002
Cr-Commit-Position: refs/heads/master@{#37310}