[strong] Make strong 'this' optional for experimentation

src/preparser.h

 // Copyright 2012 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.
 
 #ifndef V8_PREPARSER_H
 #define V8_PREPARSER_H
 
 #include "src/v8.h"
 
 #include "src/bailout-reason.h"
@@ skipping 2046 matching lines @@
   //   TemplateLiteral
 
   int beg_pos = scanner()->peek_location().beg_pos;
   int end_pos = scanner()->peek_location().end_pos;
   ExpressionT result = this->EmptyExpression();
   Token::Value token = peek();
   switch (token) {
     case Token::THIS: {
       BindingPatternUnexpectedToken(classifier);
       Consume(Token::THIS);
-      if (is_strong(language_mode())) {
+      if (FLAG_strong_this && is_strong(language_mode())) {
         // Constructors' usages of 'this' in strong mode are parsed separately.
         // TODO(rossberg): this does not work with arrow functions yet.
         if (i::IsConstructor(function_state_->kind())) {
           ReportMessage(MessageTemplate::kStrongConstructorThis);
           *ok = false;
           break;
         }
       }
       result = this->ThisExpression(scope_, factory(), beg_pos);
       break;
@@ skipping 1768 matching lines @@
       *ok = false;
       return;
     }
     has_seen_constructor_ = true;
     return;
   }
 }
 } }  // v8::internal
 
 #endif  // V8_PREPARSER_H