Tweak toString() for better type inference

sdk/lib/async/zone.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.
 
 part of dart.async;
 
 typedef dynamic ZoneCallback();
 typedef dynamic ZoneUnaryCallback(arg);
 typedef dynamic ZoneBinaryCallback(arg1, arg2);
 
@@ skipping 25 matching lines @@
                          ZoneSpecification specification,
                          Map zoneValues);
 
 /** Pair of error and stack trace. Returned by [Zone.errorCallback]. */
 class AsyncError implements Error {
   final error;
   final StackTrace stackTrace;
 
   AsyncError(this.error, this.stackTrace);
 
-  String toString() => error.toString();
+  String toString() => '$error';
 }
 
 
 class _ZoneFunction {
   final _Zone zone;
   final Function function;
   const _ZoneFunction(this.zone, this.function);
 }
 
 /**
@@ skipping 1187 matching lines @@
                                    handleUncaughtError: errorHandler);
   }
   Zone zone = Zone.current.fork(specification: zoneSpecification,
                                 zoneValues: zoneValues);
   if (onError != null) {
     return zone.runGuarded(body);
   } else {
     return zone.run(body);
   }
 }