Raise SyntaxError on let [ starting an ExpressionStatement

ES2017 forbids the sequence of tokens "let [" in in expression statements [1].

This CL makes ParserBase report those instances as SyntaxError. It also adds a
customised error message for that, because the standard "Unexpected token" is
not applicable: "let" itself is not forbidden in those context, only the
sequence of "let [".

[1] https://tc39.github.io/ecma262/#sec-expression-statement

BUG=v8:5686

Review-Url: https://codereview.chromium.org/2694003002
Cr-Commit-Position: refs/heads/master@{#43258}

src/messages.h

@@ -640,6 +640,8 @@ 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 '%'")               \

src/parsing/parser-base.h

@@ -4935,6 +4935,12 @@ ParserBase<Impl>::ParseExpressionOrLabelledStatement(
       ReportUnexpectedToken(Next());
       *ok = false;
       return impl()->NullStatement();
+    case Token::LET:
+      if (PeekAhead() != Token::LBRACK) break;
+      impl()->ReportMessageAt(scanner()->peek_location(),
+                              MessageTemplate::kUnexpectedLexicalDeclaration);
+      *ok = false;
+      return impl()->NullStatement();
     default:
       break;
   }

test/message/let-lexical-declaration-in-single-statement.js

+// Copyright 2017 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.
+
+with ({}) let [a] = [];

test/message/let-lexical-declaration-in-single-statement.out

+*%(basename)s:5: SyntaxError: Lexical declaration cannot appear in a single-statement context
+with ({}) let [a] = [];
+          ^^^
+SyntaxError: Lexical declaration cannot appear in a single-statement context