Isolate.spawn{Uri} only reports errors asynchronously.

sdk/lib/_collection_dev/schedule_immediate.dart

Index: sdk/lib/_collection_dev/schedule_immediate.dart
diff --git a/pkg/analyzer/lib/formatter.dart b/sdk/lib/_collection_dev/schedule_immediate.dart
similarity index 58%
copy from pkg/analyzer/lib/formatter.dart
copy to sdk/lib/_collection_dev/schedule_immediate.dart
index 5139cdeae58eb035b54ad1644b6b92d99817ea7a..d03356bd86450adb875e6183677b94de8dc44578 100644
--- a/pkg/analyzer/lib/formatter.dart
+++ b/sdk/lib/_collection_dev/schedule_immediate.dart
@@ -2,7 +2,7 @@
 // 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.
 
-library formatter;
+part of dart._collection.dev;
 
-export 'package:analyzer/src/services/formatter_impl.dart'
-  show CodeFormatter, FormatterOptions, FormattedSource, CodeKind;
+/** Schedule the given callback before any other event in the event-loop. */
+external void scheduleImmediate(void callback());