Remove unnecessary coupling between Promise tests and Object.observe

test/mjsunit/es6/debug-promises/reject-uncaught-uncaught.js

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Flags: --expose-debug-as debug --allow-natives-syntax
 
 // Test debug events when we only listen to uncaught exceptions and
 // there is no catch handler for the to-be-rejected Promise.
 // We expect an Exception debug event with a promise to be triggered.
 
@@ skipping 42 matching lines @@
       if (expected_events === 0) {
         assertEquals(["resolve", "end main", "reject"], log);
       } else {
         testDone(iteration + 1);
       }
     } catch (e) {
       %AbortJS(e + "\n" + e.stack);
     }
   }
 
-  // Run testDone through the Object.observe processing loop.
-  var dummy = {};
-  Object.observe(dummy, checkResult);
-  dummy.dummy = dummy;
+  %EnqueueMicrotask(checkResult);
 }
 
 testDone(0);