Dart2js support for custom element lifecycle events

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 57c8c5941c10074b71e8bb260701df0c01e7ed4b..1b0014cf0e0fabe4b8a256552ea4d2142cece750 100644
--- a/pkg/shadow_dom/lib/src/platform/patches-shadowdom-polyfill.js
+++ b/pkg/shadow_dom/lib/src/platform/patches-shadowdom-polyfill.js
@@ -30,8 +30,10 @@
       // TODO(jmesserly): do we still need these?
       if (obj instanceof NodeList) return 'NodeList';
       if (obj instanceof ShadowRoot) return 'ShadowRoot';
-      if (obj instanceof MutationRecord) return 'MutationRecord';
-      if (obj instanceof MutationObserver) return 'MutationObserver';
+      if (window.MutationRecord && (obj instanceof MutationRecord))
+          return 'MutationRecord';
+      if (window.MutationObserver && (obj instanceof MutationObserver))
+          return 'MutationObserver';
 
       var unwrapped = unwrapIfNeeded(obj);
       if (obj !== unwrapped) {