Add unhandledExceptionCallback to streamSpawnFunction.

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 2514d604162283cb10dae327665a2b876f74fa7b..410d032422039e7bea935859f541363c18f16c98 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/isolate_patch.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/isolate_patch.dart
@@ -17,8 +17,12 @@ patch class _Isolate {
   }
 
   patch static SendPort spawnFunction(void topLevelFunction(),
-      [bool UnhandledExceptionCallback(IsolateUnhandledException e)]) {
-    // TODO(9012): Don't ignore the UnhandledExceptionCallback.
+      [bool unhandledExceptionCallback(IsolateUnhandledException e)]) {
+    if (unhandledExceptionCallback != null) {
+      // TODO(9012): Implement the UnhandledExceptionCallback.
+      throw new UnimplementedError(
+          "spawnFunction with unhandledExceptionCallback");
+    }
     return IsolateNatives.spawnFunction(topLevelFunction);
   }