Add flag for disallowing for-in initializers

This does not affect use counters.

R=nikolaos@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1911963003

Cr-Commit-Position: refs/heads/master@{#35726}

src/bootstrapper.cc

@@ -2397,6 +2397,7 @@ void Bootstrapper::ExportExperimentalFromRuntime(Isolate* isolate,
 
 EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_unicode_regexps)
 EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_do_expressions)
+EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_for_in)
 EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_iterator_close)
 EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_regexp_exec)
 EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_regexp_lookbehind)
@@ -2957,6 +2958,7 @@ bool Genesis::InstallExperimentalNatives() {
   static const char* harmony_simd_natives[] = {"native harmony-simd.js",
                                                nullptr};
   static const char* harmony_do_expressions_natives[] = {nullptr};
+  static const char* harmony_for_in_natives[] = {nullptr};
   static const char* harmony_regexp_exec_natives[] = {
       "native harmony-regexp-exec.js", nullptr};
   static const char* harmony_regexp_subclass_natives[] = {nullptr};

src/flag-definitions.h

@@ -199,6 +199,7 @@ DEFINE_IMPLICATION(es_staging, move_object_start)
   V(harmony_sharedarraybuffer, "harmony sharedarraybuffer")             \
   V(harmony_simd, "harmony simd")                                       \
   V(harmony_do_expressions, "harmony do-expressions")                   \
+  V(harmony_for_in, "harmony for-in syntax")                            \
   V(harmony_regexp_property, "harmony unicode regexp property classes") \
   V(harmony_string_padding, "harmony String-padding methods")
 

src/parsing/parser.cc

@@ -3502,6 +3502,18 @@ Statement* Parser::ParseForStatement(ZoneList<const AstRawString*>* labels,
         if (!IsLexicalVariableMode(parsing_result.descriptor.mode) &&
             decl.pattern->IsVariableProxy() && decl.initializer != nullptr) {
           ++use_counts_[v8::Isolate::kForInInitializer];
+          if (FLAG_harmony_for_in) {
+            // TODO(rossberg): This error is not currently generated in the
+            // preparser, because that would lose some of the use counts
+            // recorded above. Once either the use counter or the flag is
+            // removed, the preparser should be adjusted.
+            ParserTraits::ReportMessageAt(
+                parsing_result.first_initializer_loc,
+                MessageTemplate::kForInOfLoopInitializer,
+                ForEachStatement::VisitModeString(mode));
+            *ok = false;
+            return nullptr;
+          }
           const AstRawString* name =
               decl.pattern->AsVariableProxy()->raw_name();
           VariableProxy* single_var = scope_->NewUnresolved(

test/mjsunit/harmony/for-in.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-for-in
+
+assertThrows("for (var x = 0 in {});", SyntaxError);
+assertThrows("for (const x = 0 in {});", SyntaxError);
+assertThrows("for (let x = 0 in {});", SyntaxError);