[es6] Tail calls support.

src/parsing/parser.cc

 // 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.
 
 #include "src/parsing/parser.h"
 
 #include "src/api.h"
 #include "src/ast/ast.h"
 #include "src/ast/ast-expression-rewriter.h"
 #include "src/ast/ast-expression-visitor.h"
@@ skipping 2785 matching lines @@
       Expression* is_undefined = factory()->NewCompareOperation(
           Token::EQ_STRICT, assign,
           factory()->NewUndefinedLiteral(RelocInfo::kNoPosition), pos);
 
       // is_undefined ? this : is_object_conditional
       return_value = factory()->NewConditional(
           is_undefined, ThisExpression(scope_, factory(), pos),
           is_object_conditional, pos);
     }
 
-    return_value->MarkTail();
+    // ES6 14.6.1 Static Semantics: IsInTailPosition
+    if (FLAG_harmony_tailcalls && !is_sloppy(language_mode())) {
+      return_value->MarkTail();
+    }
   }
   ExpectSemicolon(CHECK_OK);
 
   if (is_generator()) {
     Expression* generator = factory()->NewVariableProxy(
         function_state_->generator_object_variable());
     Expression* yield = factory()->NewYield(
         generator, return_value, Yield::kFinal, loc.beg_pos);
     result = factory()->NewExpressionStatement(yield, loc.beg_pos);
   } else {
@@ skipping 2919 matching lines @@
     auto class_literal = value->AsClassLiteral();
     if (class_literal->raw_name() == nullptr) {
       class_literal->set_raw_name(name);
     }
   }
 }
 
 
 }  // namespace internal
 }  // namespace v8