Isolate.spawn{Uri} only reports errors asynchronously.

runtime/lib/async_patch.dart

Index: runtime/lib/async_patch.dart
diff --git a/pkg/shadow_dom/lib/src/platform/platform-init.js b/runtime/lib/async_patch.dart
similarity index 64%
copy from pkg/shadow_dom/lib/src/platform/platform-init.js
copy to runtime/lib/async_patch.dart
index 4e83afc8c5fafb541d0e44d97fbd3621ddba5040..3299bc987df1279f92d213bee357c9793f74a90e 100644
--- a/pkg/shadow_dom/lib/src/platform/platform-init.js
+++ b/runtime/lib/async_patch.dart
@@ -2,4 +2,5 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-var Platform = {};
+import "dart:_collection-dev" show ScheduleImmediateClosure,
+                                   scheduleImmediateClosure;