Isolate.spawn{Uri} only reports errors asynchronously.

sdk/lib/_internal/lib/collection_dev_patch.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // 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.
 
 import 'dart:_js_primitives' show printString;
+import 'dart:_isolate_helper' show TimerImpl;
 
 patch class Symbol implements core.Symbol {
   patch const Symbol(String name)
       : this._name = name;
 }
 
 patch void printToConsole(String line) {
   printString('$line');
 }
+
+patch void scheduleImmediate(void callback()) {
+  // TODO(9002): don't use the Timer to enqueue the immediate callback.
+  new TimerImpl(0, callback);
+}