[gardening] Fix dart2js chrome observatory test.

Follow-up to 6a6b9ac
TEST=ci

Change-Id: Ib3bbb53a06820ab75b059e96358e0b6c6076d211
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/427302
Commit-Queue: Alexander Aprelev <[email protected]>
Reviewed-by: Ryan Macnak <[email protected]>

Author: aam

runtime/observatory/tests/observatory_ui/app_test.dart

@@ -15,7 +15,7 @@ main() async {
       print('${rec.level.name}: ${rec.time}: ${rec.message}');
     });
     Logger.root.info('Starting Observatory');
-    document.body!.children
-        .insert(0, new ObservatoryApplicationElement.created().element);
+    document.body!
+        ..appendChild(new ObservatoryApplicationElement.created().element);
   });
 }