Upgrade Symbol implementation to match current ES6 behavior.

src/stub-cache.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 263 matching lines @@
     check = BOOLEAN_CHECK;
   }
 
   if (check != RECEIVER_MAP_CHECK &&
       !function->IsBuiltin() &&
       function->shared()->is_classic_mode()) {
     // Calling non-strict non-builtins with a value as the receiver
     // 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);
   Handle<Object> probe(stub_holder->map()->FindInCodeCache(*name, flags),
                        isolate_);
   if (probe->IsCode()) return Handle<Code>::cast(probe);
 
   CallStubCompiler compiler(isolate_, argc, kind, extra_state, cache_holder);
   Handle<Code> code =
       compiler.CompileCallConstant(object, holder, name, check, function);
@@ skipping 1607 matching lines @@
           Handle<FunctionTemplateInfo>(
               FunctionTemplateInfo::cast(signature->receiver()));
     }
   }
 
   is_simple_api_call_ = true;
 }
 
 
 } }  // namespace v8::internal