fix ShadowDOM on browsers with non-native template

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

Index: pkg/shadow_dom/lib/src/platform/patches-shadowdom-polyfill.js
diff --git a/pkg/shadow_dom/lib/src/platform/patches-shadowdom-polyfill.js b/pkg/shadow_dom/lib/src/platform/patches-shadowdom-polyfill.js
index 1b0014cf0e0fabe4b8a256552ea4d2142cece750..4bc5a56318356ed5ef132910a0a5a60eb364af3a 100644
--- a/pkg/shadow_dom/lib/src/platform/patches-shadowdom-polyfill.js
+++ b/pkg/shadow_dom/lib/src/platform/patches-shadowdom-polyfill.js
@@ -35,6 +35,13 @@
       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.