diff --git a/longtask-timing/longtask-in-externalscript.html b/longtask-timing/longtask-in-externalscript.html
index ad194ac9719113..0d8f5ccb67d052 100644
--- a/longtask-timing/longtask-in-externalscript.html
+++ b/longtask-timing/longtask-in-externalscript.html
@@ -10,37 +10,50 @@
 <h1>Long Task: External Script</h1>
 <div id="log"></div>
 <script>
-  async_test(function (t) {
-    assert_implements(window.PerformanceLongTaskTiming, 'Longtasks are not supported.');
+promise_test(function(t) {
+  assert_implements(window.PerformanceLongTaskTiming, 'Longtasks are not supported.');
+  const performanceObserverTimeout = 5 * 1000;
+  const longTasksPromise = new Promise(resolve => {
     const observer = new PerformanceObserver(
-        t.step_func(function (entryList) {
-            const entries = entryList.getEntries();
-            assert_equals(entries.length, 1,
-                'Exactly one entry is expected.');
-            const longtask = entries[0];
-            checkLongTaskEntry(longtask);
-            // Assert the TaskAttributionTiming entry in attribution.
-            assert_equals(longtask.attribution.length, 1,
-                'Exactly one attribution entry is expected');
-            const attribution = longtask.attribution[0];
-            assert_equals(attribution.entryType, 'taskattribution');
-            assert_equals(attribution.name, 'unknown');
-            assert_equals(attribution.duration, 0);
-            assert_equals(attribution.startTime, 0);
-            assert_equals(attribution.containerType, 'window');
-            assert_equals(attribution.containerId, '');
-            assert_equals(attribution.containerName, '');
-            assert_equals(attribution.containerSrc, '');
-            observer.disconnect();
-            t.done();
+      t.step_func(function(entryList) {
+        const entries = entryList.getEntries();
+        entries.forEach(longtask => {
+          assert_equals(longtask.entryType, 'longtask');
+          if (hasUnrelatedTaskName(longtask.name, 'self')) {
+            return;
+          }
+          checkLongTaskEntry(longtask);
+          // Assert the TaskAttributionTiming entry in attribution.
+          assert_equals(longtask.attribution.length, 1,
+            'Exactly one attribution entry is expected');
+          const attribution = longtask.attribution[0];
+          assert_equals(attribution.entryType, 'taskattribution');
+          assert_equals(attribution.name, 'unknown');
+          assert_equals(attribution.duration, 0);
+          assert_equals(attribution.startTime, 0);
+          assert_equals(attribution.containerType, 'window');
+          assert_equals(attribution.containerId, '');
+          assert_equals(attribution.containerName, '');
+          assert_equals(attribution.containerSrc, '');
+          observer.disconnect();
+          resolve();
         })
-    );
-    observer.observe({entryTypes: ['longtask']});
-    window.onload = () => {
-        const script = document.createElement('script');
-        script.src = 'resources/makelongtask.js';
-        document.body.appendChild(script);
-    }
+      }));
+    observer.observe({
+      entryTypes: ['longtask']
+    });
+    const script = document.createElement('script');
+    script.src = 'resources/makelongtask.js';
+    document.body.appendChild(script);
+  });
+  const timeout = new Promise(
+    (resolve, reject) => t.step_timeout(
+      () => {
+        reject(new Error('observer failed to find any entries'))
+      },
+      performanceObserverTimeout)
+  )
+  return Promise.race([longTasksPromise, timeout]);
 }, 'Performance longtask entries are observable.');
 </script>
 </body>