use .append instead of .nodes.add

tests/html/mutationobserver_test.dart

Index: tests/html/mutationobserver_test.dart
diff --git a/tests/html/mutationobserver_test.dart b/tests/html/mutationobserver_test.dart
index 9710310fc239c4861e320806315a29d657c7eb01..f73f2a5bdea9293ac14074c02898b6f52cf5151c 100644
--- a/tests/html/mutationobserver_test.dart
+++ b/tests/html/mutationobserver_test.dart
@@ -67,8 +67,8 @@ main() {
             mutationCallback(2, orderedEquals([div1, div2])));
         mutationObserver.observe(container, childList: true);
 
-        container.nodes.add(div1);
-        container.nodes.add(div2);
+        container.append(div1);
+        container.append(div2);
       }, expectation);
     });
 
@@ -81,8 +81,8 @@ main() {
             new MutationObserver(mutationCallback(1, orderedEquals([div1])));
         mutationObserver.observe(container, childList: true);
 
-        container.nodes.add(div1);
-        div1.nodes.add(div2);
+        container.append(div1);
+        div1.append(div2);
       }, expectation);
     });
 
@@ -95,8 +95,8 @@ main() {
             mutationCallback(2, orderedEquals([div1, div2])));
         mutationObserver.observe(container, childList: true, subtree: true);
 
-        container.nodes.add(div1);
-        div1.nodes.add(div2);
+        container.append(div1);
+        div1.append(div2);
       }, expectation);
     });