Isolate.spawn{Uri} only reports errors asynchronously.

runtime/lib/async_sources.gypi

Index: runtime/lib/async_sources.gypi
diff --git a/runtime/lib/async_sources.gypi b/runtime/lib/async_sources.gypi
index 896e5088c2a6242804a35c1cf72431d03b6f8efa..f1319b96933bbc3f894425356d7d546787cf9d51 100644
--- a/runtime/lib/async_sources.gypi
+++ b/runtime/lib/async_sources.gypi
@@ -5,6 +5,8 @@
 # This file contains all sources for the dart:async library.
 {
   'sources': [
+    'async_patch.dart',
+    # The above file needs to be first as it imports required libraries.
     'deferred_load_patch.dart',
     'schedule_microtask_patch.dart',
     'timer_patch.dart',