Remove duplicated code from comma-separated Expression parsing

src/messages.h

Index: src/messages.h
diff --git a/src/messages.h b/src/messages.h
index cf49ac9c5cb817218b3c9abb08f0e4e9ad607b64..661c66f90836977a427cb03f628bd3144e8e952f 100644
--- a/src/messages.h
+++ b/src/messages.h
@@ -454,8 +454,6 @@ class CallSiteUtils : public AllStatic {
   T(NoCatchOrFinally, "Missing catch or finally after try")                    \
   T(NotIsvar, "builtin %%IS_VAR: not a variable")                              \
   T(ParamAfterRest, "Rest parameter must be last formal parameter")            \
-  T(InvalidRestParameter,                                                      \
-    "Rest parameter must be an identifier or destructuring pattern")           \

[adamk, 2016/08/26 20:13:03]

I don't think there's much lost for users in losing this message. We didn't emit
it in non-arrow functions anyway.

[Dan Ehrenberg, 2016/08/26 22:48:53]

This doesn't make me all that happy. The old message was better. Could we extend
the better message to the non-arrow function case?

[nickie, 2016/08/29 08:42:59]

In principle I would agree with Dan.  But this error is only reported very
rarely and even in those cases, the message is misleading.  All the following
wrong arrow functions produce different error messages (unexpected something or
other):

d8> f = (...42) => 42;
d8> f = (x, ...42) => 42;
d8> f = (...[42]) => 42;
d8> f = (x, ...[42]) => 42;
d8> f = (...[x+1]) => 42;
d8> f = (a, ...[x+1]) => 42;

I'm not saying that these error messages are good, only that Adam's change does
not affect these cases.

Only things like this produce InvalidRestParameter:

d8> f = (...x = []) => 42;
(d8):1: SyntaxError: Rest parameter must be an identifier or destructuring
pattern
f = (...x = []) => 42;
     ^^^^^^^^^
SyntaxError: Rest parameter must be an identifier or destructuring pattern

As I see it, the rest parameter is "x" and it is indeed an identifier.  The
problem is that it carries an initializer and that is forbidden.  I think the
error message is not good anyway.

   T(PushPastSafeLength,                                                        \
     "Pushing % elements on an array-like of length % "                         \
     "is disallowed, as the total surpasses 2**53-1")                           \