Add second argument to Future error handlers.

sdk/lib/async/async_error.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.
 
 part of dart.async;
 
 final Expando _stackTraceExpando = new Expando("asynchronous error");
 
 void _attachStackTrace(o, st) {
   if (o == null || o is bool || o is num || o is String) return;
   _stackTraceExpando[o] = st;
 }
 
 /**
  * *This is an experimental API.*
  *
  * Get the [StackTrace] attached to [o].
  *
  * If object [o] was thrown and caught in a dart:async method, a [StackTrace]
  * object was attached to it. Use [getAttachedStackTrace] to get that object.
  *
  * Returns [null] if no [StackTrace] was attached.
  */
 getAttachedStackTrace(o) {
   if (o == null || o is bool || o is num || o is String) return null;
   return _stackTraceExpando[o];
 }
+
+// TODO(floitsch): should we make this an Error class? Probably depends, if we
+// want to make the class public.

[Lasse Reichstein Nielsen, 2013/10/04 09:17:39]

We can implement Error without problems. We have the stackTrace getter, so it's
fine.

[floitsch, 2013/10/05 17:22:59]

Done.

+class _AsyncError {
+  final error;
+  final StackTrace stackTrace;
+
+  _AsyncError(this.error, this.stackTrace);
+}
+
+class _UncaughtAsyncError extends _AsyncError {
+  _UncaughtAsyncError(error, StackTrace stackTrace) : super(error, stackTrace);
+
+  String toString() {
+    String result = "Uncaught Error: ${error}";
+    var trace = stackTrace;
+    if (trace == null) trace = getAttachedStackTrace(error);

[Lasse Reichstein Nielsen, 2013/10/04 09:17:39]

This looks wrong to me. If anything, the attached stack trace should be detached
when the object is created, not when the toString is called, and it should only
be used if an explicit stacktrace isn't provided in the constructor.
Something like:
  _UncaughtAsyncError(error, StackTrace stackTrace) 
    : super(error, stackTrace != null ? stackTrace :
getAttachedStackTrace(error)) {
     _attachStackTrace(error, null);
  }

Then we can do:
  if (trace == null && error is Error) trace = error.stackTrace;
to get at the third way a stack trace can be attached.

[floitsch, 2013/10/05 17:22:59]

done.

+
+    // Clear the attached stack trace.
+    _attachStackTrace(error, null);
+
+    if (trace != null) {
+      result += "\nStack Trace:\n$trace";
+    }
+    return result;
+  }
+}