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

pkg/web_components/test/created_watcher_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.
+
+library template_wrappers_test;
+
+import 'dart:html';
+import 'dart:async';
+import 'dart:js' show context, JsObject;
+import 'package:unittest/html_config.dart';
+import 'package:unittest/unittest.dart';
+import 'package:web_components/created_watcher.dart';
+
+main() {
+  useHtmlConfiguration();
+
+  test('created_watcher is supported', () {
+    expect(isSupported, isTrue);
+  });
+
+  test('no events for previously upgraded elements', () {
+    var sA = watch('x-a').listen(expectAsync((e) {
+      fail('this should not be called');
+    }, count: 0));
+    var sB = watch('x-b', extendsTag: 'div').listen(expectAsync((e) {
+      fail('this should not be called');
+    }, count: 0));
+    var sC = watch('x-c').listen(expectAsync((e) {
+      fail('this should not be called');
+    }, count: 0));
+    return new Future.value().then((_) {
+      sA.cancel();
+      sB.cancel();
+      sC.cancel();
+    });
+  });
+
+  test('events seen when registration was done before', () {
+    var sA = watch('x-a').listen(expectAsync((e) {
+      expect(e is HtmlElement, isTrue);
+      expect(new JsObject.fromBrowserObject(e)['x'], 2);
+    }));
+
+    var sB = watch('x-b', extendsTag: 'div').listen(expectAsync((e) {
+      expect(new JsObject.fromBrowserObject(e)['x'], 3);
+    }));
+
+    var sC = watch('x-c').listen(expectAsync((e) {
+      fail('x-c is not registered yet');
+    }, count: 0));
+
+    context.callMethod('addA');
+    context.callMethod('addB');
+
+    return new Future.value().then((_) {
+      sA.cancel();
+      sB.cancel();
+      sC.cancel();
+    });
+  });
+
+  test('events seen when registration is done afterwards', () {
+    int lastValue;
+    var sC = watch('x-c').listen(expectAsync((e) {
+      lastValue = new JsObject.fromBrowserObject(e)['x'];
+    }, count: 2));
+
+    context.callMethod('registerC');
+    expect(lastValue, 4);
+    context.callMethod('addC');
+    expect(lastValue, 5);
+    return new Future.value().then((_) {
+      sC.cancel();
+    });
+  });
+}