According to the ES6 specification, in "for in/of" loops like:

   for (var v of [1,2,3]) return f(...);

the call to f() should not be considered a tail call.  This was
not working properly, i.e., the case without declarations:

   var v;
   for (v of [1,2,3]) return f(...);

R=adamk@chromium.org, ishell@chromium.org
BUG=
LOG=N

Review-Url: https://codereview.chromium.org/2343823002
Cr-Commit-Position: refs/heads/master@{#39497}