Raise SyntaxError on let [ starting an ExpressionStatement

src/messages.h

 // Copyright 2006-2008 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // The infrastructure used for (localized) message reporting in V8.
 //
 // Note: there's a big unresolved issue about ownership of the data
 // structures used by this framework.
 
 #ifndef V8_MESSAGES_H_
@@ skipping 620 matching lines @@
   T(UnexpectedReserved, "Unexpected reserved word")                            \
   T(UnexpectedStrictReserved, "Unexpected strict mode reserved word")          \
   T(UnexpectedSuper, "'super' keyword unexpected here")                        \
   T(UnexpectedNewTarget, "new.target expression is not allowed here")          \
   T(UnexpectedTemplateString, "Unexpected template string")                    \
   T(UnexpectedToken, "Unexpected token %")                                     \
   T(UnexpectedTokenIdentifier, "Unexpected identifier")                        \
   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 '%'")               \
   T(UnterminatedArgList, "missing ) after argument list")                      \
   T(UnterminatedRegExp, "Invalid regular expression: missing /")               \
   T(UnterminatedTemplate, "Unterminated template literal")                     \
   T(UnterminatedTemplateExpr, "Missing } in template expression")              \
   T(FoundNonCallableHasInstance, "Found non-callable @@hasInstance")           \
   T(InvalidHexEscapeSequence, "Invalid hexadecimal escape sequence")           \
   T(InvalidUnicodeEscapeSequence, "Invalid Unicode escape sequence")           \
@@ skipping 78 matching lines @@
                                         const MessageLocation* loc,
                                         Handle<Object> message_obj,
                                         v8::Local<v8::Value> api_exception_obj);
 };
 
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_MESSAGES_H_