[es2015] Precompute the descriptive string for symbols.

src/objects.h

 // Copyright 2015 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_OBJECTS_H_
 #define V8_OBJECTS_H_
 
 #include <iosfwd>
 #include <memory>
 
@@ skipping 7100 matching lines @@
   DISALLOW_IMPLICIT_CONSTRUCTORS(Name);
 };
 
 
 // ES6 symbols.
 class Symbol: public Name {
  public:
   // [name]: The print name of a symbol, or undefined if none.
   DECL_ACCESSORS(name, Object)
 
+  // [descriptive_string]: The descriptive string of a symbol.  This is the
+  // value returned from the Symbol.prototype.toString method.
+  DECL_ACCESSORS(descriptive_string, String)
+
   DECL_INT_ACCESSORS(flags)
 
   // [is_private]: Whether this is a private symbol.  Private symbols can only
   // be used to designate own properties of objects.
   DECL_BOOLEAN_ACCESSORS(is_private)
 
   // [is_well_known_symbol]: Whether this is a spec-defined well-known symbol,
   // or not. Well-known symbols do not throw when an access check fails during
   // a load.
   DECL_BOOLEAN_ACCESSORS(is_well_known_symbol)
 
   // [is_public]: Whether this is a symbol created by Symbol.for. Calling
   // Symbol.keyFor on such a symbol simply needs to return the attached name.
   DECL_BOOLEAN_ACCESSORS(is_public)
 
   DECLARE_CAST(Symbol)
 
   // Dispatched behavior.
   DECLARE_PRINTER(Symbol)
   DECLARE_VERIFIER(Symbol)
 
   // Layout description.
   static const int kNameOffset = Name::kSize;
-  static const int kFlagsOffset = kNameOffset + kPointerSize;
+  static const int kDescriptiveStringOffset = kNameOffset + kPointerSize;
+  static const int kFlagsOffset = kDescriptiveStringOffset + kPointerSize;
   static const int kSize = kFlagsOffset + kPointerSize;
 
   // Flags layout.
   static const int kPrivateBit = 0;
   static const int kWellKnownSymbolBit = 1;
   static const int kPublicBit = 2;
 
   typedef FixedBodyDescriptor<kNameOffset, kFlagsOffset, kSize> BodyDescriptor;
 
   void SymbolShortPrint(std::ostream& os);
@@ skipping 2288 matching lines @@
   }
 };
 
 
 }  // NOLINT, false-positive due to second-order macros.
 }  // NOLINT, false-positive due to second-order macros.
 
 #include "src/objects/object-macros-undef.h"
 
 #endif  // V8_OBJECTS_H_