[es6] Tail calls support.

src/ast/ast.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_AST_AST_H_
 #define V8_AST_AST_H_
 
 #include "src/assembler.h"
 #include "src/ast/ast-value-factory.h"
 #include "src/ast/modules.h"
@@ skipping 1977 matching lines @@
   BailoutId LookupId() const { return BailoutId(local_id(2)); }
   BailoutId CallId() const { return BailoutId(local_id(3)); }
 
   bool is_uninitialized() const {
     return IsUninitializedField::decode(bit_field_);
   }
   void set_is_uninitialized(bool b) {
     bit_field_ = IsUninitializedField::update(bit_field_, b);
   }
 
-  bool is_tail() const { return IsTailField::decode(bit_field_); }
+  TailCallMode tail_call_mode() const {
+    return IsTailField::decode(bit_field_) ? TailCallMode::kAllow
+                                           : TailCallMode::kDisallow;
+  }
   void MarkTail() override {
     bit_field_ = IsTailField::update(bit_field_, true);
   }
 
   enum CallType {
     POSSIBLY_EVAL_CALL,
     GLOBAL_CALL,
     LOOKUP_SLOT_CALL,
     NAMED_PROPERTY_CALL,
     KEYED_PROPERTY_CALL,
@@ skipping 1540 matching lines @@
   // the parser-level zone.
   Zone* parser_zone_;
   AstValueFactory* ast_value_factory_;
 };
 
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_AST_AST_H_