Make Map.set() and Set.add() chainable

src/runtime.cc

Index: src/runtime.cc
diff --git a/src/runtime.cc b/src/runtime.cc
index eba17a19437b86de7c502faf8f1f3d99cf2ba8f8..8e59a0c940d1e1de0747c6c874154752c105bf4f 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -1548,7 +1548,7 @@ RUNTIME_FUNCTION(Runtime_SetAdd) {
   Handle<OrderedHashSet> table(OrderedHashSet::cast(holder->table()));
   table = OrderedHashSet::Add(table, key);
   holder->set_table(*table);
-  return isolate->heap()->undefined_value();
+  return *holder;
 }
 
 
@@ -1686,7 +1686,7 @@ RUNTIME_FUNCTION(Runtime_MapSet) {
   Handle<OrderedHashMap> table(OrderedHashMap::cast(holder->table()));
   Handle<OrderedHashMap> new_table = OrderedHashMap::Put(table, key, value);
   holder->set_table(*new_table);
-  return isolate->heap()->undefined_value();
+  return *holder;
 }
 
 
@@ -1800,7 +1800,7 @@ RUNTIME_FUNCTION(Runtime_WeakCollectionSet) {
   RUNTIME_ASSERT(table->IsKey(*key));
   Handle<ObjectHashTable> new_table = ObjectHashTable::Put(table, key, value);
   weak_collection->set_table(*new_table);
-  return isolate->heap()->undefined_value();
+  return *weak_collection;
 }