Re-land CL 9466047.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 7d8a55e7dec15bdfde69103b84c667e3bd07d0e7..482b7944644361cd50f0728cc064b5a6dcd0e5fa 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -2589,6 +2589,20 @@ class DescriptorArray: public FixedArray {
   static const int kMaxNumberOfDescriptors = 1024 + 512;
 
  private:
+  // An entry in a DescriptorArray, represented as an (array, index) pair.
+  class Entry {
+   public:
+    inline explicit Entry(DescriptorArray* descs, int index) :
+        descs_(descs), index_(index) { }
+
+    inline PropertyType type() { return descs_->GetType(index_); }
+    inline Object* GetCallbackObject() { return descs_->GetValue(index_); }
+
+   private:
+    DescriptorArray* descs_;
+    int index_;
+  };
+
   // Conversion from descriptor number to array indices.
   static int ToKeyIndex(int descriptor_number) {
     return descriptor_number+kFirstIndex;
@@ -7922,6 +7936,10 @@ class AccessorPair: public Struct {
     }
   }
 
+  bool ContainsAccessor() {
+    return IsJSAccessor(getter()) || IsJSAccessor(setter());
+  }
+
 #ifdef OBJECT_PRINT
   void AccessorPairPrint(FILE* out = stdout);
 #endif
@@ -7934,6 +7952,15 @@ class AccessorPair: public Struct {
   static const int kSize = kSetterOffset + kPointerSize;
 
  private:
+  // Strangely enough, in addition to functions and harmony proxies, the spec
+  // requires us to consider undefined as a kind of accessor, too:
+  //    var obj = {};
+  //    Object.defineProperty(obj, "foo", {get: undefined});
+  //    assertTrue("foo" in obj);
+  bool IsJSAccessor(Object* obj) {
+    return obj->IsSpecFunction() || obj->IsUndefined();
+  }
+
   DISALLOW_IMPLICIT_CONSTRUCTORS(AccessorPair);
 };