Make arguments of MutationObserver named.

lib/html/dartium/html_dartium.dart

Index: lib/html/dartium/html_dartium.dart
diff --git a/lib/html/dartium/html_dartium.dart b/lib/html/dartium/html_dartium.dart
index 78da89f43d42be3df450c1f01a54be25184bbac3..9c41146fae5e95a228f9c84460f53ae4672cfa6b 100644
--- a/lib/html/dartium/html_dartium.dart
+++ b/lib/html/dartium/html_dartium.dart
@@ -22825,14 +22825,14 @@ abstract class MutationObserver {
   List<MutationRecord> takeRecords();
 
   void observe(Node target,
-               [Map options,
+               {Map options,
                 bool childList,
                 bool attributes,
                 bool characterData,
                 bool subtree,
                 bool attributeOldValue,
                 bool characterDataOldValue,
-                List<String> attributeFilter]);
+                List<String> attributeFilter});
 }
 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
@@ -22847,14 +22847,14 @@ class _MutationObserverImpl extends NativeFieldWrapperClass1 implements Mutation
   List<MutationRecord> takeRecords() native "MutationObserver_takeRecords_Callback";
 
   void observe(Node target,
-               [Map options,
+               {Map options,
                 bool childList,
                 bool attributes,
                 bool characterData,
                 bool subtree,
                 bool attributeOldValue,
                 bool characterDataOldValue,
-                List<String> attributeFilter]) {
+                List<String> attributeFilter}) {
 
     // Parse options into map of known type.
     var parsedOptions = _createDict();
@@ -43557,7 +43557,7 @@ class _MutationObserverScheduler extends _MeasurementScheduler {
     // so we just make a dummy event and listen for that.
     _observer = new MutationObserver(this._handleMutation);
     _dummy = new DivElement();
-    _observer.observe(_dummy, {}, attributes: true);
+    _observer.observe(_dummy, attributes: true);
   }
 
   void _schedule() {