Don't attempt to transition JSProxy to an observed map / Object.getNotifier() must SetObserved

src/runtime.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 13910 matching lines @@
 RUNTIME_FUNCTION(MaybeObject*, Runtime_SetIsObserved) {
   SealHandleScope shs(isolate);
   ASSERT(args.length() == 1);
   CONVERT_ARG_CHECKED(JSReceiver, obj, 0);
   if (obj->IsJSGlobalProxy()) {
     Object* proto = obj->GetPrototype();
     if (proto->IsNull()) return isolate->heap()->undefined_value();
     ASSERT(proto->IsJSGlobalObject());
     obj = JSReceiver::cast(proto);
   }
+  if (obj->IsJSProxy())
+    return isolate->heap()->undefined_value();
+
   ASSERT(!(obj->map()->is_observed() && obj->IsJSObject() &&
            JSObject::cast(obj)->HasFastElements()));
   ASSERT(obj->IsJSObject());
   return JSObject::cast(obj)->SetObserved(isolate);
 }
 
 
 RUNTIME_FUNCTION(MaybeObject*, Runtime_SetObserverDeliveryPending) {
   SealHandleScope shs(isolate);
   ASSERT(args.length() == 0);
@@ skipping 217 matching lines @@
     // Handle last resort GC and make sure to allow future allocations
     // to grow the heap without causing GCs (if possible).
     isolate->counters()->gc_last_resort_from_js()->Increment();
     isolate->heap()->CollectAllGarbage(Heap::kNoGCFlags,
                                        "Runtime::PerformGC");
   }
 }
 
 
 } }  // namespace v8::internal