Isolate.spawn{Uri} only reports errors asynchronously.

runtime/lib/isolate_patch.dart

 // Copyright (c) 2012, 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:collection" show HashMap;
 
 patch class ReceivePort {
   /* patch */ factory ReceivePort() = _ReceivePortImpl;
 
   /* patch */ factory ReceivePort.fromRawReceivePort(RawReceivePort rawPort) =
@@ skipping 223 matching lines @@
       // The VM will invoke [_startIsolate] with entryPoint as argument.
       SendPort controlPort = _spawnFunction(entryPoint);
       RawReceivePort readyPort = new RawReceivePort();
       controlPort.send([readyPort.sendPort, message]);
       readyPort.handler = (readyMessage) {
         assert(readyMessage == 'started');
         readyPort.close();
         completer.complete(new Isolate._fromControlPort(controlPort));
       };
     } catch(e, st) {
-      // TODO(14718): we want errors to go into the returned future.
-      rethrow;
+      // Don't complete immediately with an error, since the caller didn't
+      // yet have the opportunity to install an error handler.
+      scheduleMicrotask(() {
+        completer.completeError(e, st);
+      });

[Lasse Reichstein Nielsen, 2013/12/13 11:16:58]

How about doing the same as in the JS patch:
 try { 
    ...
    Completer completer = new ...
    readyPort.handler = ...
    return completer.future;
 } catch (e, st) {
    return new Future<Isolate>.error(e, st);
 }

Add space after "catch" here and below.

[floitsch, 2013/12/13 12:13:37]

Done.

     };
     return completer.future;
   }
 
   /* patch */ static Future<Isolate> spawnUri(
       Uri uri, List<String> args, var message) {
     Completer completer = new Completer<Isolate>.sync();
     try {
       // The VM will invoke [_startIsolate] and not `main`.
       SendPort controlPort = _spawnUri(uri.toString());
       RawReceivePort readyPort = new RawReceivePort();
       controlPort.send([readyPort.sendPort, args, message]);
       readyPort.handler = (readyMessage) {
         assert(readyMessage == 'started');
         readyPort.close();
         completer.complete(new Isolate._fromControlPort(controlPort));
       };
     } catch(e, st) {
-      // TODO(14718): we want errors to go into the returned future.
-      rethrow;
+      // Don't complete immediately with an error, since the caller didn't
+      // yet have the opportunity to install an error handler.
+      scheduleMicrotask(() {
+        completer.completeError(e, st);
+      });
     };
     return completer.future;
   }
 
   static final RawReceivePort _self = _mainPort;
   static RawReceivePort get _mainPort native "Isolate_mainPort";
 
   static SendPort _spawnFunction(Function topLevelFunction)
       native "Isolate_spawnFunction";
 
   static SendPort _spawnUri(String uri) native "Isolate_spawnUri";
 }