Add annotations on native fields and methods

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

Index: sdk/lib/html/templates/html/dart2js/factoryprovider_MutationObserver.darttemplate
diff --git a/sdk/lib/html/templates/html/dart2js/factoryprovider_MutationObserver.darttemplate b/sdk/lib/html/templates/html/dart2js/factoryprovider_MutationObserver.darttemplate
index d92979b3943fa7acc996993f44a2d37b267efef1..affcb7c5569ac4c85ab71ca3d5b09d1506a35b32 100644
--- a/sdk/lib/html/templates/html/dart2js/factoryprovider_MutationObserver.darttemplate
+++ b/sdk/lib/html/templates/html/dart2js/factoryprovider_MutationObserver.darttemplate
@@ -5,20 +5,10 @@
 part of html;
 
 class $FACTORYPROVIDER {
-  static $CONSTRUCTOR create$(CONSTRUCTOR)(MutationCallback callback) {
 
-    // This is a hack to cause MutationRecord to appear to be instantiated.
-    //
-    // MutationCallback has a parameter type List<MutationRecord>.  From this we
-    // infer a list is created in the browser, but not the element type, because
-    // other native fields and methods return plain List which is too general
-    // and would imply creating anything.  This statement is a work-around.
-    JS('MutationRecord','0');
-
-    return _create$(CONSTRUCTOR)(callback);
-  }
-
-  static $CONSTRUCTOR _create$(CONSTRUCTOR)(MutationCallback callback) native '''
+  @Creates('MutationObserver')
+  @Creates('MutationRecord')
+  static $CONSTRUCTOR create$(CONSTRUCTOR)(MutationCallback callback) native '''
     var constructor =
         window.MutationObserver || window.WebKitMutationObserver ||
         window.MozMutationObserver;