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

pkg/web_components/test/interop_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/interop.dart';
+
+main() {
+  useHtmlConfiguration();
+  registerDartType('x-a', XAWrapper);
+  registerDartType('x-b', XBWrapper, extendsTag: 'div');
+  registerDartType('x-c', XCWrapper);
+
+  test('interop is supported', () {
+    expect(isSupported, isTrue);
+  });
+
+  test('previously created elements are not upgraded', () {
+    var a = document.querySelector('x-a');
+    expect(a is HtmlElement, isTrue, reason: 'x-a is HtmlElement');
+    expect(a is XAWrapper, isFalse, reason: 'x-a should not be upgraded yet');
+    expect(_readX(a), 0);
+
+    var b = document.querySelector('[is=x-b]');
+    expect(b is DivElement, isTrue, reason: 'x-b is DivElement');
+    expect(b is XBWrapper, isFalse, reason: 'x-b should not be upgraded yet');
+    expect(_readX(b), 1);
+
+    var c = document.querySelector('x-c');
+    expect(c is HtmlElement, isTrue, reason: 'x-c is HtmlElement');
+    expect(c is XCWrapper, isFalse, reason: 'x-c should not be upgraded yet');
+    expect(_readX(c), null, reason: 'x-c has not been registered in JS yet');
+  });
+
+  test('events seen for anything created after registering Dart type', () {
+    context.callMethod('addA');
+    var list = document.querySelectorAll('x-a');
+    expect(list.length, 2);
+    var a = list[1];
+    expect(a is HtmlElement, isTrue, reason: 'x-a is HtmlElement');
+    expect(a is XAWrapper, isTrue, reason: 'x-a is upgraded to XAWrapper');
+    expect(a.x, 2);
+    expect(a.wrapperCount, 0);
+
+    context.callMethod('addB');
+    list = document.querySelectorAll('[is=x-b]');
+    expect(list.length, 2);
+    var b = list[1];
+    expect(b is DivElement, isTrue, reason: 'x-b is DivElement');
+    expect(b is XBWrapper, isTrue, reason: 'x-b is upgraded to XBWrapper');
+    expect(b.x, 3);
+    expect(b.wrapperCount, 1);
+  });
+
+  test('events seen if Dart type is registered before registerElement', () {
+    var c = document.querySelector('x-c');
+    expect(c is XCWrapper, isFalse);
+    expect(_readX(c), null, reason: 'x-c has not been registered in JS yet');
+
+    context.callMethod('registerC');
+    c = document.querySelector('x-c');
+    expect(c is XCWrapper, isTrue);
+    expect(c.x, 4);
+    expect(c.wrapperCount, 2);
+
+    context.callMethod('addC');
+    var list = document.querySelectorAll('x-c');
+    expect(list.length, 2);
+    expect(list[0], c);
+    c = list[1];
+    expect(c is HtmlElement, isTrue, reason: 'x-c is HtmlElement');
+    expect(c is XCWrapper, isTrue, reason: 'x-c is upgraded to XCWrapper');
+    expect(c.x, 5);
+    expect(c.wrapperCount, 3);
+  });
+}
+int _count = 0;
+
+abstract class Wrapper {
+  int wrapperCount = _count++;
+  int get x => _readX(this);
+}
+
+_readX(e) => new JsObject.fromBrowserObject(e)['x'];
+
+class XAWrapper extends HtmlElement with Wrapper {
+  XAWrapper.created() : super.created();
+}
+
+class XBWrapper extends DivElement with Wrapper {
+  XBWrapper.created() : super.created();
+}
+
+class XCWrapper extends HtmlElement with Wrapper {
+  XCWrapper.created() : super.created();
+}