Add a flag for legacy const semantics

src/parser.cc

Index: src/parser.cc
diff --git a/src/parser.cc b/src/parser.cc
index d795b3ce166ae54af87e623553386faf888cc25f..55b4122d71f8cbcccc2d04e8c5d494ef01e508a1 100644
--- a/src/parser.cc
+++ b/src/parser.cc
@@ -919,6 +919,7 @@ Parser::Parser(ParseInfo* info)
   set_allow_harmony_spread_arrays(FLAG_harmony_spread_arrays);
   set_allow_harmony_new_target(FLAG_harmony_new_target);
   set_allow_strong_mode(FLAG_strong_mode);
+  set_allow_legacy_const(FLAG_legacy_const);
   for (int feature = 0; feature < v8::Isolate::kUseCounterFeatureCount;
        ++feature) {
     use_counts_[feature] = 0;
@@ -1379,16 +1380,21 @@ Statement* Parser::ParseStatementListItem(bool* ok) {
       }
       return ParseClassDeclaration(NULL, ok);
     case Token::CONST:
+      if (allow_const()) {
+        return ParseVariableStatement(kStatementListItem, NULL, ok);
+      }
+      break;
     case Token::VAR:
       return ParseVariableStatement(kStatementListItem, NULL, ok);
     case Token::LET:
       if (is_strict(language_mode())) {
         return ParseVariableStatement(kStatementListItem, NULL, ok);
       }
-      // Fall through.
+      break;
     default:
-      return ParseStatement(NULL, ok);
+      break;
   }
+  return ParseStatement(NULL, ok);
 }
 
 
@@ -1937,7 +1943,7 @@ Statement* Parser::ParseSubStatement(ZoneList<const AstRawString*>* labels,
       // In ES6 CONST is not allowed as a Statement, only as a
       // LexicalDeclaration, however we continue to allow it in sloppy mode for
       // backwards compatibility.
-      if (is_sloppy(language_mode())) {
+      if (is_sloppy(language_mode()) && allow_legacy_const()) {
         return ParseVariableStatement(kStatement, NULL, ok);
       }
 
@@ -2426,13 +2432,14 @@ void Parser::ParseVariableDeclarations(VariableDeclarationContext var_context,
       return;
     }
     Consume(Token::VAR);
-  } else if (peek() == Token::CONST) {
+  } else if (peek() == Token::CONST && allow_const()) {
     Consume(Token::CONST);
-    if (is_sloppy(language_mode())) {
+    if (is_sloppy(language_mode()) && allow_legacy_const()) {
       parsing_result->descriptor.mode = CONST_LEGACY;
       parsing_result->descriptor.init_op = Token::INIT_CONST_LEGACY;
       ++use_counts_[v8::Isolate::kLegacyConst];
     } else {
+      DCHECK(is_strict(language_mode()));
       DCHECK(var_context != kStatement);
       parsing_result->descriptor.mode = CONST;
       parsing_result->descriptor.init_op = Token::INIT_CONST;
@@ -3490,7 +3497,7 @@ Statement* Parser::ParseForStatement(ZoneList<const AstRawString*>* labels,
   bool is_let_identifier_expression = false;
   DeclarationParsingResult parsing_result;
   if (peek() != Token::SEMICOLON) {
-    if (peek() == Token::VAR || peek() == Token::CONST ||
+    if (peek() == Token::VAR || (peek() == Token::CONST && allow_const()) ||
         (peek() == Token::LET && is_strict(language_mode()))) {
       ParseVariableDeclarations(kForStatement, &parsing_result, CHECK_OK);
       is_const = parsing_result.descriptor.mode == CONST;
@@ -3531,8 +3538,7 @@ Statement* Parser::ParseForStatement(ZoneList<const AstRawString*>* labels,
         Block* init_block = nullptr;
 
         // special case for legacy for (var/const x =.... in)
-        if (is_sloppy(language_mode()) &&
-            !IsLexicalVariableMode(parsing_result.descriptor.mode) &&
+        if (!IsLexicalVariableMode(parsing_result.descriptor.mode) &&
             parsing_result.declarations[0].initializer != nullptr) {
           VariableProxy* single_var = scope_->NewUnresolved(
               factory(), parsing_result.SingleName(), Variable::NORMAL,
@@ -3615,8 +3621,7 @@ Statement* Parser::ParseForStatement(ZoneList<const AstRawString*>* labels,
         }
 
         // Create a TDZ for any lexically-bound names.
-        if (is_strict(language_mode()) &&
-            IsLexicalVariableMode(parsing_result.descriptor.mode)) {
+        if (IsLexicalVariableMode(parsing_result.descriptor.mode)) {
           DCHECK_NULL(init_block);
 
           init_block =