Raise SyntaxError on let [ starting an ExpressionStatement

src/messages.h

Index: src/messages.h
diff --git a/src/messages.h b/src/messages.h
index 433d04469d28c5791050bc579da8a194d1f8a9d4..5c8f71a1fd5ce789da119488b4e99bedec61042f 100644
--- a/src/messages.h
+++ b/src/messages.h
@@ -638,6 +638,7 @@ class ErrorUtils : public AllStatic {
   T(UnexpectedTokenNumber, "Unexpected number")                                \
   T(UnexpectedTokenString, "Unexpected string")                                \
   T(UnexpectedTokenRegExp, "Unexpected regular expression")                    \
+  T(UnexpectedTokenLetLBrack, "Unexpected token sequence let [")               \

[Dan Ehrenberg, 2017/02/16 09:16:51]

From a user's perspective, the error they made was putting a lexical declaration
as the body of something expecting a single statement. I like SpiderMonkey's
error here: " lexical declarations can't appear in single-statement context:"  .
Note that we already have a pretty ugly error for "with ({}) let x = 1;"
("unexpected identifier"), so arguably cleaning up the messages here could be a
task for another patch.

[vabr (Chromium), 2017/02/16 17:09:33]

Thanks, that's a great point.
I adopted the message from SpiderMonkey you suggested, it makes sense to do this
right when adding this as new.

I agree with you about the other error as well, and will do a separate patch for
that, to allow selective reverting if needed.

   T(UnknownLabel, "Undefined label '%'")                                       \
   T(UnresolvableExport,                                                        \
     "The requested module does not provide an export named '%'")               \