Fix issue 965.

src/v8natives.js

Index: src/v8natives.js
diff --git a/src/v8natives.js b/src/v8natives.js
index 50a2774d122072654cabadc5b8e0a9d8a4f81921..894df1402598cadb5b93ac06da0075feb9d45d4f 100644
--- a/src/v8natives.js
+++ b/src/v8natives.js
@@ -615,7 +615,13 @@ function DefineOwnProperty(obj, p, desc, should_throw) {
     } else {
       flag |= READ_ONLY;
     }
-    %DefineOrRedefineDataProperty(obj, p, desc.getValue(), flag);
+    var value;  // Default value is undefined.
+    if (desc.hasValue()) {
+      value = desc.getValue();
+    } else if (!IS_UNDEFINED(current)) {
+      value = current.getValue();
+    }
+    %DefineOrRedefineDataProperty(obj, p, value, flag);
   } else {
     if (desc.hasGetter() && IS_FUNCTION(desc.getGet())) {
        %DefineOrRedefineAccessorProperty(obj, p, GETTER, desc.getGet(), flag);