[strong] Make strong 'this' optional for experimentation

R=arv@chromium.org, conradw@chromium.org
BUG=v8:3956
LOG=N

Review URL: https://codereview.chromium.org/1180943007

Cr-Commit-Position: refs/heads/master@{#29026}

src/flag-definitions.h

@@ -174,6 +174,7 @@ DEFINE_IMPLICATION(use_strong, use_strict)
 
 DEFINE_BOOL(strong_mode, false, "experimental strong language mode")
 DEFINE_IMPLICATION(use_strong, strong_mode)
+DEFINE_BOOL(strong_this, true, "don't allow 'this' to escape from constructors")
 
 DEFINE_BOOL(es_staging, false, "enable all completed harmony features")
 DEFINE_BOOL(harmony, false, "enable all completed harmony features")

src/parser.cc

@@ -2562,6 +2562,8 @@ Statement* Parser::ParseExpressionOrLabelledStatement(
       return nullptr;
 
     case Token::THIS:
+      if (!FLAG_strong_this) break;
+      // Fall through.
     case Token::SUPER:
       if (is_strong(language_mode()) &&
           i::IsConstructor(function_state_->kind())) {

src/preparser.cc

@@ -599,6 +599,8 @@ PreParser::Statement PreParser::ParseExpressionOrLabelledStatement(bool* ok) {
       return Statement::Default();
 
     case Token::THIS:
+      if (!FLAG_strong_this) break;
+      // Fall through.
     case Token::SUPER:
       if (is_strong(language_mode()) &&
           i::IsConstructor(function_state_->kind())) {

src/preparser.h

@@ -2064,7 +2064,7 @@ ParserBase<Traits>::ParsePrimaryExpression(ExpressionClassifier* classifier,
     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())) {