Add second argument to Future error handlers.

sdk/lib/async/async_error.dart

Index: sdk/lib/async/async_error.dart
diff --git a/sdk/lib/async/async_error.dart b/sdk/lib/async/async_error.dart
index 05fbb88b211b769782bcc8d43639fdbc52ff98c5..6cea8a57d013addc5831ccb0d129e14697e56030 100644
--- a/sdk/lib/async/async_error.dart
+++ b/sdk/lib/async/async_error.dart
@@ -25,3 +25,30 @@ 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;
+  }
+}