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

pkg/web_components/lib/created_watcher.js

Index: pkg/web_components/lib/created_watcher.js
diff --git a/pkg/web_components/lib/created_watcher.js b/pkg/web_components/lib/created_watcher.js
new file mode 100644
index 0000000000000000000000000000000000000000..ff9db337ba3dfc5e4b5e888e3a065149376c7f4c
--- /dev/null
+++ b/pkg/web_components/lib/created_watcher.js
@@ -0,0 +1,69 @@
+// Copyright (c) 2014, 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.
+
+// Updates document.registerElement so Dart can see when Javascript custom
+// elements are created, and wrap them to provide a Dart friendly API.
+(function (doc) {

[Jennifer Messerly, 2014/06/09 17:36:07]

can we include this in dart_support.js? all polymer.dart programs will need
it... and we're moving platform.js and dart_support calls out of polymer.html.

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

I debated about it earlier :)  - will do when we are done with the review so
that the diffs stay in place.

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

sounds good!

+  var registeredWatchers = {};
+  var originalRegisterElement = doc.registerElement;
+  if (!originalRegisterElement) {
+    throw new Error('document.registerElement is not present.');
+  }
+
+  function registerElement(name, options) {
+    var proto, extendsOption;
+    if (options !== undefined) {
+      proto = options.prototype;
+    } else {
+      proto = Object.create(HTMLElement.prototype);
+      options = {protoptype: proto};
+    }
+
+    var original = proto.createdCallback;
+    var newCallback = function() {
+      original.call(this);
+      var name = this.getAttribute('is') || this.tagName;

[Jennifer Messerly, 2014/06/09 17:36:07]

fyi: CE polyfill uses localName and toLowerCase instead. Not sure if it makes a
difference
https://github.com/Polymer/CustomElements/blob/master/src/CustomElements.js#L392
https://github.com/Polymer/CustomElements/blob/master/src/CustomElements.js#L337

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

Done.

+      var watchers = registeredWatchers[name.toUpperCase()];
+      if (!watchers) return;
+      for (var i in watchers) {

[Jennifer Messerly, 2014/06/09 17:36:07]

based on my other suggestion, it will be possible to remove ids :)

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

Done.

+        watchers[i](this);
+      }
+    };
+
+    var descriptor = Object.getOwnPropertyDescriptor(proto, 'createdCallback');
+    if (!descriptor || descriptor.writable) {
+      proto.createdCallback = newCallback;
+    } else if (descriptor.configurable) {
+      descriptor['value'] = newCallback;
+      Object.defineProperty(proto, 'createdCallback', descriptor);
+    } else {
+      console.warn("Couldn't patch prototype to notify Dart when " + name +

[Jennifer Messerly, 2014/06/09 17:36:07]

console.error?

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

Done.

+          " elements are created. This can be fixed by making the " +
+          "createdCallback in " + name + " a configurable property.");
+    }
+    return originalRegisterElement.call(doc, name, options);

[Jennifer Messerly, 2014/06/09 17:36:07]

this, name, options?

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

Done.

+  }
+
+  function registerElementCreatedWatcher(name, watcherId, watcher) {
+    if (!watcher) return;
+    name = name.toUpperCase();
+    var watchers = registeredWatchers[name];
+    if (!watchers) {
+      registeredWatchers[name] = watchers = [];
+    }
+    watchers[watcherId] = watcher;
+  }
+
+  function unregisterElementCreatedWatcher(name, watcherId) {
+    name = name.toUpperCase();
+    var watchers = registeredWatchers[name];
+    if (watchers) {
+      delete watchers[watcherId];
+    }
+  }
+
+  doc.registerElementCreatedWatcher = registerElementCreatedWatcher;

[Jennifer Messerly, 2014/06/09 17:36:07]

it might be worth keeping these private, and only consumed by the Dart APIs.

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

Done.

+  doc.unregisterElementCreatedWatcher = unregisterElementCreatedWatcher;
+  doc.registerElement = registerElement;
+})(document);