fix ShadowDOM on browsers with non-native template

pkg/shadow_dom/lib/src/platform/patches-shadowdom-polyfill.js

 /*
  * Copyright 2013 The Polymer Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style
  * license that can be found in the LICENSE file.
  */
 (function() {
   var ShadowDOMPolyfill = window.ShadowDOMPolyfill;
   var wrap = ShadowDOMPolyfill.wrap;
 
   // patch in prefixed name
@@ skipping 17 matching lines @@
     var unwrapIfNeeded = ShadowDOMPolyfill.unwrapIfNeeded;
     function getTag(obj) {
       // TODO(jmesserly): do we still need these?
       if (obj instanceof NodeList) return 'NodeList';
       if (obj instanceof ShadowRoot) return 'ShadowRoot';
       if (window.MutationRecord && (obj instanceof MutationRecord))
           return 'MutationRecord';
       if (window.MutationObserver && (obj instanceof MutationObserver))
           return 'MutationObserver';
 
+      // TODO(jmesserly): this prevents incorrect interaction between ShadowDOM
+      // and dart:html's <template> polyfill. Essentially, ShadowDOM is
+      // polyfilling native template, but our Dart polyfill fails to detect this
+      // because the unwrapped node is an HTMLUnknownElement, leading it to
+      // think the node has no content.
+      if (obj instanceof HTMLTemplateElement) return 'HTMLTemplateElement';
+
       var unwrapped = unwrapIfNeeded(obj);
       if (obj !== unwrapped) {
         // Fix up class names for Firefox.
         // For some of them (like HTMLFormElement and HTMLInputElement),
         // the "constructor" property of the unwrapped nodes points at the
         // same constructor as the wrapper.
         var ctor = obj.constructor
         if (ctor === unwrapped.constructor) {
           var name = ctor._ShadowDOMPolyfill$cacheTag_;
           if (!name) {
             name = Object.prototype.toString.call(unwrapped);
             name = name.substring(8, name.length - 1);
             ctor._ShadowDOMPolyfill$cacheTag_ = name;
           }
           return name;
         }
 
         obj = unwrapped;
       }
       return originalGetTag(obj);
     }
 
     return getTag;
   };
 })();