Remove unnecessary coupling between Promise tests and Object.observe

test/mjsunit/es6/debug-promises/throw-with-undefined-reject.js

Index: test/mjsunit/es6/debug-promises/throw-with-undefined-reject.js
diff --git a/test/mjsunit/es6/debug-promises/throw-with-undefined-reject.js b/test/mjsunit/es6/debug-promises/throw-with-undefined-reject.js
index 94dcdffa225e11a5255801e8d9eccfdd972e50f7..c88feb9c397080974240980ca4ae5dfd853d951d 100644
--- a/test/mjsunit/es6/debug-promises/throw-with-undefined-reject.js
+++ b/test/mjsunit/es6/debug-promises/throw-with-undefined-reject.js
@@ -79,10 +79,7 @@ function testDone(iteration) {
     }
   }
 
-  // Run testDone through the Object.observe processing loop.
-  var dummy = {};
-  Object.observe(dummy, checkResult);
-  dummy.dummy = dummy;
+  %EnqueueMicrotask(checkResult);
 }
 
 testDone(0);