Async functions are implemented via special generator functions; special in the
sense that they generally do not immediately yield.  However, our generators
implementation still assumed that every generator function initially yields
(concretely: before doing the state dispatch in a loop header).  This CL fixes
that.

R=littledan@chromium.org, rmcilroy@chromium.org
BUG=chromium:638019

Review-Url: https://codereview.chromium.org/2253033002
Cr-Commit-Position: refs/heads/master@{#38684}