[builtins] Add an initial fast-path to Object.assign.

src/lookup.h

Index: src/lookup.h
diff --git a/src/lookup.h b/src/lookup.h
index 7275c57640eba8ad656146bd24e091d977926763..48048bb51dc33a9721a9328a143345cb84a1638b 100644
--- a/src/lookup.h
+++ b/src/lookup.h
@@ -238,6 +238,7 @@ class LookupIterator final BASE_EMBEDDED {
   }
   bool IsConfigurable() const { return property_details().IsConfigurable(); }
   bool IsReadOnly() const { return property_details().IsReadOnly(); }
+  bool IsEnumerable() const { return property_details().IsEnumerable(); }
   Representation representation() const {
     return property_details().representation();
   }