[es6] support rest parameters in arrow functions (alternative)

[es6] support rest parameters in arrow functions (alternative)

BUG=v8:2159
LOG=N
R=marja@chromium.org, arv@chromium.org, dslomov@chromium.org

[caitp (gmail), 2015-03-11 05:58:27]

Alternative version of https://crrev.com/987083003 --- this one doesn't change
the parser to incorporate an extra lookahead token (which _may_ be preferable).

Some issues:
  - preparser is never able to figure out if arrow functions contain duplicate
parameters, which (I think?) prevents test-parsing from doing much good when
errors are expected.

  - right now, `var x = (...a);` is not an error. It should be, but using this
strategy I'm not sure yet how to make sure it's treated as an error.

  - the errors you do get are generally not showing very helpful text

I think the version which relies on an extra lookahead is probably a better
option, but think about it

[marja, 2015-03-11 18:15:44]

Yeah, preparser doesn't check duplicates in arrow function param lists, since
when we start parsing the param list we don't yet know that it's a param list...
(this could maybe be solved with some speculative checking, idk).

I hadn't had time to look at this CL yet, arv@ probably will also have a look
since you also brought up this alternative to the lookahead increase.

[arv (Not doing code reviews), 2015-03-11 18:59:13]

https://codereview.chromium.org/997823003/diff/1/src/preparser.h
File src/preparser.h (right):

https://codereview.chromium.org/997823003/diff/1/src/preparser.h#newcode2005
src/preparser.h:2005: case Token::ELLIPSIS:
Beware of fall through here.

https://codereview.chromium.org/997823003/diff/1/src/preparser.h#newcode2406
src/preparser.h:2406: // TODO(caitp): the error is an unexpected `...`, it would
be nice if we
Can you store the position before calling ParseConditionalExpression?

https://codereview.chromium.org/997823003/diff/1/src/preparser.h#newcode2416
src/preparser.h:2416: // TODO(caitp): report an error if an unexpected `...`
occurred.
Here it would be good to have an ast ;-)

I'm not really sure how to best track this. Potentially we could pass in a
Location pointer but it gets ugly.