Support extending TemplateElement in IE11 + webcomponents-lite.js

tests/html/custom/document_register_template_test.dart

Index: tests/html/custom/document_register_template_test.dart
diff --git a/tests/html/custom/document_register_template_test.dart b/tests/html/custom/document_register_template_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..11b0b8da0c3ba839818aab1b14ccaf8faca21b90
--- /dev/null
+++ b/tests/html/custom/document_register_template_test.dart
@@ -0,0 +1,24 @@
+import 'package:unittest/unittest.dart';
+import 'package:unittest/html_config.dart';
+import 'dart:html';
+import '../utils.dart';
+
+main() {
+  useHtmlConfiguration();
+
+  setUp(() => customElementsReady);
+
+  test('can register custom template with webcomponents-lite polyfill', () {
+    document.registerElement('my-element', MyElement, extendsTag: 'template');
+    var e = new Element.tag('template', 'my-element');
+    document.body.append(e);
+    expect(e is TemplateElement, isTrue);
+    expect(e.method(), 'value');
+  });
+}
+
+
+class MyElement extends TemplateElement {
+  MyElement.created() : super.created();
+  method() => 'value';
+}