Use a transformer to initialize custom elements and call initMethod. This is the

pkg/polymer/test/event_path_declarative_test.dart

 // Copyright (c) 2013, 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.
 
 // This is ported from event-path-declarative-test.dart in polymer/test/html/.
 // While the original test was intended to test event.path support, we changed
 // the test structure just to check that the event was handled in the expected
 // order.
 library polymer.test.event_path_declarative_test;
 
@@ skipping 63 matching lines @@
 
   divTestEventHandler(e, detail, sender) {
     _observedEvents.add(sender);
   }
 
   testEventHandler(e, detail, sender) {
     _observedEvents.add(sender);
   }
 }
 
-@initMethod _init() {
+@initMethod init() {
   useHtmlConfiguration();
   // TODO(sigmund): switch back to use @CustomTag. We seem to be running into a
   // problem where using @CustomTag doesn't guarantee that we register the tags
   // in the following order (the query from mirrors is non deterministic).
   // We shouldn't care about registration order though. See dartbug.com/14459
   Polymer.register('x-zug', XZug);
   Polymer.register('x-foo', XFoo);
   Polymer.register('x-bar', XBar);
 
   _testFired = new Completer();
@@ skipping 16 matching lines @@
       var expectedPath = [ xBarContent, xBarDiv, xFooContent,
           xZugContent, xZugDiv, xZug, xFooDiv, xFoo, xBar];
       debugName(e) => '${e.localName}#${e.id}';
       expect(_observedEvents, expectedPath, reason:
         '<br>\nexpected: ${expectedPath.map(debugName).join(',')}'
         '<br>\nactual: ${_observedEvents.map(debugName).join(',')}'
         );
     });
   });
 }