Support extending TemplateElement in IE11 + webcomponents-lite.js

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index 2043449707e8e4baed88c8153fea0097a0bc187b..05fc09a5052ce3fc62606e0113185202a960e1f9 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -42882,8 +42882,11 @@ class _JSElementUpgrader implements ElementUpgrader {
       _nativeType = HtmlElement;
     } else {
       var element = document.createElement(extendsTag);
-      if (!JS('bool', '(# instanceof window[#])',
-          element, baseClassName)) {
+      if (!JS('bool', '(# instanceof window[#])', element, baseClassName) &&
+        // Exception to support template elements (extended for core pieces of
+        // Polymer 1.0) when using the webcomponents-lite.js polyfill on IE11:
+        !((extendsTag == 'template' &&
+         JS('bool', '(# instanceof window["HTMLUnknownElement"])', element)))) {
         throw new UnsupportedError(
             'extendsTag does not match base native class');
       }