Add parser support for generators.

Add parser support for generators.

This patchset begins by adding support for "yield", which is unlike other tokens
in JS.  In a generator, whether strict or classic, it is a syntactic keyword.
In classic mode it is an identifier.  In strict mode it is reserved.

This patch adds YIELD as a token to the scanner, and adapts the preparser and
parser appropriately.  It also parses "function*", indicating that a function is
actually a generator, for both eagerly and lazily parsed functions.

Currently "yield" just compiles as "return".

BUG=
TESTS=Added new test suite for parsing.

[wingo, 2013-03-28 15:27:52]

This is a rebased version of https://codereview.chromium.org/12646003/ with a
better description.  The only change from that CL is resolving a conflict in
flag-definitions.h.  Uploading here so that I can work on trunk for my next
patch that depends on this one.