In commit b3bfc0bd, I corrected the source
position of yield-exceptions by not setting the "return position" on returns
that correspond to yields. It turns out that this caused a bug with debug
stepping. The proper fix is to keep the return position on those returns but
additionally attach the yield's source position to the Throw emitted in
VisitYield.

R=rmcilroy@chromium.org, yangguo@chromium.org
BUG=v8:4907

Review-Url: https://codereview.chromium.org/2051783002
Cr-Commit-Position: refs/heads/master@{#36896}