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

pkg/polymer/lib/src/build/common.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.
 
 /** Common methods used by transfomers. */
 library polymer.src.build.common;
 
 import 'dart:async';
 import 'dart:math' show min, max;
 
@@ skipping 99 matching lines @@
         checkDocType: samePackage && options.isHtmlEntryPoint(id));
     });
   }
 
   Future<bool> assetExists(AssetId id, Transform transform) =>
       transform.getInput(id).then((_) => true).catchError((_) => false);
 
   String toString() => 'polymer ($runtimeType)';
 }
 
-/** Create an [AssetId] for a [url] seen in the [source] asset. */
+/**
+ * Create an [AssetId] for a [url] seen in the [source] asset. By default this
+ * is used to resolve relative urls that occur in HTML assets, including
+ * cross-package urls of the form "packages/foo/bar.html". Dart-style "package:"
+ * urls are not resolved unless [allowPackageColonUrls] is set.
+ */
 // TODO(sigmund): delete once this is part of barback (dartbug.com/12610)
-AssetId resolve(AssetId source, String url, TransformLogger logger, Span span) {
+AssetId resolve(AssetId source, String url, TransformLogger logger, Span span,

[Jennifer Messerly, 2014/02/03 19:09:32]

instead of allowPackageColonUrls, is there a way we could build this as two
functions? e.g. resolveDartUri and resolveHtmlUri?

alternatively maybe just rename it "dartUris"

btw, great doc comment :)

[Siggi Cherem (dart-lang), 2014/02/04 00:20:35]

We could. This also gave me another idea. I can check whether
source.path.endsWith('.dart') then allow 'package:' automatically in that case.
Thoughts?

[Jennifer Messerly, 2014/02/04 00:37:40]

Love it!

+    {bool allowPackageColonUrls: false}) {
   if (url == null || url == '') return null;
   var uri = Uri.parse(url);
   var urlBuilder = path.url;
   if (uri.host != '' || uri.scheme != '' || urlBuilder.isAbsolute(url)) {
+    if (allowPackageColonUrls && uri.scheme == 'package') {
+      var index = uri.path.indexOf('/');
+      if (index != -1) {
+        return new AssetId(uri.path.substring(0, index),
+            'lib${uri.path.substring(index)}');
+      }
+    } 
+
     logger.error('absolute paths not allowed: "$url"', span: span);
     return null;
   }
 
   var targetPath = urlBuilder.normalize(
       urlBuilder.join(urlBuilder.dirname(source.path), url));
   var segments = urlBuilder.split(targetPath);
   var sourceSegments = urlBuilder.split(source.path);
   assert (sourceSegments.length > 0);
   var topFolder = sourceSegments[0];
@@ skipping 84 matching lines @@
   return builder.relative(builder.join('/', id.path),
       from: builder.join('/', builder.dirname(sourceId.path)));
 }
 
 
 /** Convert system paths to asset paths (asset paths are posix style). */
 String _systemToAssetPath(String assetPath) {
   if (path.Style.platform != path.Style.windows) return assetPath;
   return path.posix.joinAll(path.split(assetPath));
 }