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

[Jennifer Messerly, 2014/06/09 17:28:48]

2014?

[Siggi Cherem (dart-lang), 2014/06/09 19:29:01]

Done.

+// 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);
+int _sequenceId = 0;
+
+/// Returns whether [watch] is supported, which requires to have
+/// `created_watcher.js` already loaded in the page.
+bool get isSupported => _doc.hasProperty('registerElementCreatedWatcher');
+
+/// Watches when Javascript custom elements named [tagName] are created and
+/// provides a reference to the created element in a broadcast stream.
+Stream<Element> watch(String tagName, {String extendsTag}) {

[Jennifer Messerly, 2014/06/09 17:28:48]

extendsTag looks unused -- remove?

[Siggi Cherem (dart-lang), 2014/06/09 19:29:02]

Now that we call the upgrader, we do need it :)

+  if (!isSupported) {
+    throw new UnsupportedError("Couldn't find `registerElementCreatedWatcher`. "

[Jennifer Messerly, 2014/06/09 17:28:48]

document.registerElementCreatedWatcher

[Siggi Cherem (dart-lang), 2014/06/09 19:29:02]

Done.

+        "Please make sure that `packages/web_components/created_watcher.js` is "
+        "loaded and available before calling this function.");
+  }
+
+  var id = _sequenceId++;
+  var controller = new StreamController<Element>.broadcast(sync: true,

[Jennifer Messerly, 2014/06/09 17:28:48]

eeek, this gives me a bad feeling. It feels like mutation observers reinvented.

instead of a general event mechanism, can we make it specific to upgrading
custom elements?

/// register a [dartType] given a custom element [name]
registerDartType(String name, Type dartType);

... that way it is specific to interop & avoids overhead from the Stream

[Siggi Cherem (dart-lang), 2014/06/09 19:29:01]

Done. Given the changes we made, I plan to rename this file after we are done
with the review (that way diffs are still together in rietveld).

Some ideas for a new name? I'm currently thinking `register.dart`

[Jennifer Messerly, 2014/06/09 19:33:52]

yeah sounds good to me. another idea is interop.dart

+      onCancel: () =>
+          _doc.callMethod('unregisterElementCreatedWatcher', [tagName, id]));
+  _doc.callMethod('registerElementCreatedWatcher',
+      [tagName, id, controller.add]);
+  return controller.stream;
+}