[WIP] for-of iterator finalization

[WIP] for-of iterator finalization

BUG=

[neis, 2016-02-12 12:22:23]

Description was changed from

==========
[WIP] for-of iterator finalization

BUG=
==========

to

==========
[WIP] for-of iterator finalization

BUG=
==========

[neis, 2016-02-12 12:30:13]

Ok, here is what I currently have.  The issue we discussed is still unsolved.

Moreover, some tests from our suite currently fail for unclear reasons.  One
alarming example is the do-expressions mjsunit test, which crashes when the
rewriter inserts a statement into some block when the zone appears to be
invalid.  Some or most of the failing tests are related to the interpreter.  I
had talked to Michi about the problem where the variable->IsContextSlot() check
fails and it wasn't entirely clear if there is an actual problem somewhere or if
the check is just too strong.

https://codereview.chromium.org/1695583003/diff/1/src/ast/ast.h
File src/ast/ast.h (right):

https://codereview.chromium.org/1695583003/diff/1/src/ast/ast.h#newcode906
src/ast/ast.h:906: 
Instead of having iterator_, we could extract the variable from assign_iterator
or next_result.

https://codereview.chromium.org/1695583003/diff/1/src/parsing/parser.cc
File src/parsing/parser.cc (left):

https://codereview.chromium.org/1695583003/diff/1/src/parsing/parser.cc#oldco...
src/parsing/parser.cc:5903: 
The changes to RewriteYieldStar are unrelated.

https://codereview.chromium.org/1695583003/diff/1/src/parsing/parser.cc
File src/parsing/parser.cc (right):

https://codereview.chromium.org/1695583003/diff/1/src/parsing/parser.cc#newco...
src/parsing/parser.cc:6302: void
ParserTraits::BuildIteratorClose(ZoneList<Statement*>* statements,
The changes to this BuildIteratorClose are unrelated too.  There is a separate
BuildIteratorClose further below that is used in the for-of rewriting.  I had
hoped to have only one function but merging them seems to make things look too
obfuscated.

https://codereview.chromium.org/1695583003/diff/1/test/mjsunit/harmony/iterat...
File test/mjsunit/harmony/iterator-close.js (right):

https://codereview.chromium.org/1695583003/diff/1/test/mjsunit/harmony/iterat...
test/mjsunit/harmony/iterator-close.js:27: //}, 42);
This and the other two commented out tests fail with the .catch reference error
that we discussed.