Changed to use JSInterop

tools/dom/templates/html/impl/impl_MutationObserver.darttemplate

Index: tools/dom/templates/html/impl/impl_MutationObserver.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_MutationObserver.darttemplate b/tools/dom/templates/html/impl/impl_MutationObserver.darttemplate
index 4208322b7bcb2002bc5dc0a84c191c8259693d23..3469eef1c0c86d14650a1dc3830710b1e4f4e0ba 100644
--- a/tools/dom/templates/html/impl/impl_MutationObserver.darttemplate
+++ b/tools/dom/templates/html/impl/impl_MutationObserver.darttemplate
@@ -18,6 +18,30 @@ $else
         '!!(window.MutationObserver || window.WebKitMutationObserver)');
 $endif
   }
+$if DARTIUM
+  @DocsEditable()
+$if JSINTEROP
+  static MutationObserver _create(callback) => wrap_jso(_blink.BlinkMutationObserver.instance.constructorCallback_1_((List<MutationRecord> mutations, MutationObserver observer) {
+      var wrapped_mutations = [];
+      for (var mutation in mutations) {
+        // Wrap the Javascript object for each MutationRecord.
+        wrapped_mutations.add(wrap_jso(mutation));
+      }
+      // Wrap the MutationObserver too.
+      callback(wrapped_mutations, wrap_jso(observer));
+  }));
+$else
+  static MutationObserver _create(callback) => _blink.BlinkMutationObserver.instance.constructorCallback_1_((List<MutationRecord> mutations, MutationObserver observer) {
+      var wrapped_mutations = [];
+      for (var mutation in mutations) {
+        // Wrap the Javascript object for each MutationRecord.
+        wrapped_mutations.add(wrap_jso(mutation));
+      }
+      // Wrap the MutationObserver too.
+      callback(wrapped_mutations, wrap_jso(observer));
+  });
+$endif
+$endif
 
   /**
    * Observes the target for the specified changes.