Add "created-watcher" to the web_components package. This was adapted from

pkg/web_components/lib/created_watcher.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.
+
+/// Provides notifications whenever a Javascript Custom Elements is created.
+/// This will not work unless `created_watcher.js` is loaded.
+library web_components.created_watcher;
+
+import 'dart:async' show Stream, StreamController;
+import 'dart:html' show document, Element;
+import 'dart:js' show JsObject, JsFunction;
+
+final _doc = new JsObject.fromBrowserObject(document);
+
+/// Returns whether [watch] is supported, which requires to have
+/// `created_watcher.js` already loaded in the page.

[Siggi Cherem (dart-lang), 2014/06/06 22:31:47]

one difference is that for now I'm keeping it simple and we require the code to
be there already. I'm anyways planning to include that in the polymer.html that
our users already import.

[justinfagnani, 2014/06/06 23:33:05]

sounds good, i'll be a hybrid page anyway, if someone's not using the build for
some reason.

+bool get isSupported => _doc.hasProperty('registerElementCreatedWatcher');
+
+/// Watches when Javascript custom elements named [tagName] are created and
+/// provides a reference to the created element in the returned stream.

[justinfagnani, 2014/06/06 23:33:05]

You might want to mention that it's a single-subscriber stream.

[Siggi Cherem (dart-lang), 2014/06/09 17:10:27]

I switched to broadcast, but clarified in the comment too.

+Stream<Element> watch(String tagName, {String extendsTag}) {
+  if (!isSupported) {
+    throw new UnsupportedError("Couldn't find `registerElementCreatedWatcher`. "
+        "Please make sure that `packages/web_components/created_watcher.js` is "
+        "loaded and available before calling this function.");
+  }
+
+  var controller = new StreamController<Element>(sync: true);
+  var params = {'createdCallback': new JsFunction.withThis(controller.add)};
+  if (extendsTag != null) params['extends'] = extendsTag;
+  _doc.callMethod('registerElementCreatedWatcher',
+      [tagName, new JsObject.jsify(params)]);
+  // TODO(sigmund): remove watchers when susbcriptions are cancelled.
+  return controller.stream;
+}