Upgrade Symbol implementation to match current ES6 behavior.

src/stub-cache.cc

Index: src/stub-cache.cc
diff --git a/src/stub-cache.cc b/src/stub-cache.cc
index dc2c3406a6aead526238893d801b8abc8efe565b..6b6cc5df05cc9e45c9f6bdc868c6f59621d8579f 100644
--- a/src/stub-cache.cc
+++ b/src/stub-cache.cc
@@ -281,6 +281,11 @@ Handle<Code> StubCache::ComputeCallConstant(int argc,
     // requires boxing.
     return Handle<Code>::null();
   }
+  if (check == SYMBOL_CHECK) {
+    // Boxing always required for Symbol value receivers,
+    // as even Symbol builtins expect them to be boxed.
+    return Handle<Code>::null();
+  }
 
   Code::Flags flags = Code::ComputeMonomorphicFlags(
       kind, extra_state, cache_holder, Code::FAST, argc);