Make integer indexed properties ("elements") work for proxies.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index 3e924399ac647ae9ab8e8ccd0a2bb0aeb8a7b88f..8b088faebfc695a5e666d3a191ae8f41c98a9485 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -4329,8 +4329,11 @@ bool JSObject::HasHiddenProperties() {
 }
 
 
-bool JSObject::HasElement(uint32_t index) {
-  return HasElementWithReceiver(this, index);
+bool JSReceiver::HasElement(uint32_t index) {
+  if (IsJSProxy()) {
+    return JSProxy::cast(this)->HasElementWithHandler(index);
+  }
+  return JSObject::cast(this)->HasElementWithReceiver(this, index);
 }