Convert legacy 'native code' to JS-forms in dart:html

lib/html/templates/html/dart2js/factoryprovider_MutationObserver.darttemplate

Index: lib/html/templates/html/dart2js/factoryprovider_MutationObserver.darttemplate
diff --git a/lib/html/templates/html/dart2js/factoryprovider_MutationObserver.darttemplate b/lib/html/templates/html/dart2js/factoryprovider_MutationObserver.darttemplate
index 88d29d6f28c51b4faf3f012310cf67db08c4559d..83ff8c9d7c214250dde4f21280d89a39ba1dd018 100644
--- a/lib/html/templates/html/dart2js/factoryprovider_MutationObserver.darttemplate
+++ b/lib/html/templates/html/dart2js/factoryprovider_MutationObserver.darttemplate
@@ -9,4 +9,14 @@ class $FACTORYPROVIDER {
         window.MozMutationObserver;
     return new constructor(callback);
   ''';
+
+  // TODO(sra): Dart2js inserts a conversion when a Dart function (i.e. an
+  // object with a call method) is passed to a native method.  This is so the
+  // native code sees a JavaScript function.
+  //
+  // This does not happen when a function is 'passed' to a JS-form so it is not
+  // possible to rewrite the above code to, e.g. (simplified):
+  //
+  // static createMutationObserver(MutationCallback callback) =>
+  //    JS('var', 'new (window.MutationObserver)(#)', callback);
 }