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..1ad53c40f48d034bfbb1e0ef3f78addf489f6ba4 100644
--- a/src/messages.h
+++ b/src/messages.h
@@ -638,6 +638,8 @@ class ErrorUtils : public AllStatic {
   T(UnexpectedTokenNumber, "Unexpected number")                                \
   T(UnexpectedTokenString, "Unexpected string")                                \
   T(UnexpectedTokenRegExp, "Unexpected regular expression")                    \
+  T(UnexpectedLexicalDeclaration,                                              \
+    "Lexical declaration cannot appear in a single-statement context")         \
   T(UnknownLabel, "Undefined label '%'")                                       \
   T(UnresolvableExport,                                                        \
     "The requested module does not provide an export named '%'")               \