Revert "Making all callbacks use zones"

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

 // 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
 // BSD-style license that can be found in the LICENSE file.
 
 part of $LIBRARYNAME;
 
 $(ANNOTATIONS)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
 $!MEMBERS
   /**
    * Checks to see if the mutation observer API is supported on the current
@@ skipping 66 matching lines @@
   @JSName('observe')
   void _call(target, options) native;
 
   factory MutationObserver(MutationCallback callback) {
     // Dummy statement to mark types as instantiated.
     JS('MutationObserver|MutationRecord', '0');
 
     return JS('MutationObserver',
         'new(window.MutationObserver||window.WebKitMutationObserver||'
         'window.MozMutationObserver)(#)',
-        convertDartClosureToJS(_wrapZone(callback), 2));
+        convertDartClosureToJS(callback, 2));
   }
-$else
-  factory MutationObserver(MutationCallback callback) =>
-      new MutationObserver._(_wrapZone(callback));
 $endif
 }