Remove unnecessary coupling between Promise tests and Object.observe

test/mjsunit/es6/debug-promises/throw-with-undefined-reject.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 an exception is thrown inside a Promise, which is
 // caught by a custom promise, which has no reject handler.
 // We expect two Exception debug events:
 //  1) when the exception is thrown in the promise q.
@@ skipping 61 matching lines @@
       if (expected_events === 0) {
         assertEquals(["resolve", "construct", "end main", "throw caught"], 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);