Add failing global error handling test.

sdk/lib/_internal/compiler/implementation/lib/isolate_patch.dart

Index: sdk/lib/_internal/compiler/implementation/lib/isolate_patch.dart
diff --git a/sdk/lib/_internal/compiler/implementation/lib/isolate_patch.dart b/sdk/lib/_internal/compiler/implementation/lib/isolate_patch.dart
index aab847a4dc3a3579b582c2270be9cfc3e17adc1c..2514d604162283cb10dae327665a2b876f74fa7b 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/isolate_patch.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/isolate_patch.dart
@@ -18,6 +18,7 @@ patch class _Isolate {
 
   patch static SendPort spawnFunction(void topLevelFunction(),
       [bool UnhandledExceptionCallback(IsolateUnhandledException e)]) {
+    // TODO(9012): Don't ignore the UnhandledExceptionCallback.
     return IsolateNatives.spawnFunction(topLevelFunction);
   }