[proxies] Recognize arraylike proxies in Object.prototype.toString.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index bdc484dfd4b3746a566861cb1666c73248168f2f..1749a6909d116174e9934bcd007b741925dde045 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -1,4 +1,4 @@
-// Copyright 2012 the V8 project authors. All rights reserved.
+// 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.
 
@@ -1359,6 +1359,10 @@ class Object {
 
   inline void VerifyApiCallResultType();
 
+  // ES6 19.1.3.6 Object.prototype.toString
+  MUST_USE_RESULT static MaybeHandle<String> ObjectProtoToString(
+      Isolate* isolate, Handle<Object> object);
+
   // Prints this object without details.
   void ShortPrint(FILE* out = stdout);
 
@@ -1887,6 +1891,10 @@ class JSReceiver: public HeapObject {
   // Returns the class name ([[Class]] property in the specification).
   String* class_name();
 
+  // Returns the builtin string tag used in Object.prototype.toString.
+  MUST_USE_RESULT static MaybeHandle<String> BuiltinStringTag(
+      Handle<JSReceiver> object);
+
   // Returns the constructor name (the name (possibly, inferred name) of the
   // function that was used to instantiate the object).
   static Handle<String> GetConstructorName(Handle<JSReceiver> receiver);
@@ -2472,10 +2480,6 @@ class JSObject: public JSReceiver {
   static bool AllCanRead(LookupIterator* it);
   static bool AllCanWrite(LookupIterator* it);
 
-  // ES6 19.1.3.6 Object.prototype.toString
-  MUST_USE_RESULT static MaybeHandle<String> ObjectProtoToString(
-      Isolate* isolate, Handle<Object> object);
-
  private:
   friend class JSReceiver;
   friend class Object;