Check for let in lexically bound names for short object literals

An identifier may be parsed in an object literal like {let}, but
this was previously left out of lexical name checking. This patch
adds that check to prohibit code like
  let {let} = {let: 1}

BUG=v8:4403
LOG=N
R=adamk

Review URL: https://codereview.chromium.org/1401253003

Cr-Commit-Position: refs/heads/master@{#31278}

src/preparser.h

@@ -2649,6 +2649,10 @@ ParserBase<Traits>::ParsePropertyDefinition(
           scanner()->FindSymbol(classifier->duplicate_finder(), 1) != 0) {
         classifier->RecordDuplicateFormalParameterError(scanner()->location());
       }
+      if (name_token == Token::LET) {
+        classifier->RecordLetPatternError(
+            scanner()->location(), MessageTemplate::kLetInLexicalBinding);
+      }
 
       ExpressionT lhs = this->ExpressionFromIdentifier(
           name, next_beg_pos, next_end_pos, scope_, factory());

test/message/let-lexical-name-in-object-prohibited.js

+// Copyright 2015 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.
+//
+// Flags: --harmony-sloppy --harmony-sloppy-let --harmony-destructuring
+
+let {let};

test/message/let-lexical-name-in-object-prohibited.out

+*%(basename)s:7: SyntaxError: let is disallowed as a lexically bound name
+let {let};
+     ^^^
+SyntaxError: let is disallowed as a lexically bound name
+