Add an error callback to StackZoneSpecification.

pkg/stack_trace/README.md

 This library provides the ability to parse, inspect, and manipulate stack traces
 produced by the underlying Dart implementation. It also provides functions to
 produce string representations of stack traces in a more readable format than
 the native [StackTrace] implementation.
 
 `Trace`s can be parsed from native [StackTrace]s using `Trace.from`, or captured
 using `Trace.current`. Native [StackTrace]s can also be directly converted to
 human-readable strings using `Trace.format`.
 
 [StackTrace]: http://api.dartlang.org/docs/releases/latest/dart_core/StackTrace.html
@@ skipping 187 matching lines @@
     ===== asynchronous gap ===========================
     dart:async                                _Future.then
     test.dart 13:12                           scheduleAsync
     test.dart 7:18                            main.<fn>
     package:stack_trace/src/chain.dart 93:20  Chain.capture
     test.dart 6:16                            main
     dart:isolate                              _RawReceivePortImpl._handleMessage
 
 That's a lot easier to understand!
 
-### `Chain.track`
-
-For the most part `Chain.capture` will notice when an error is thrown and
-associate the correct stack chain with it. However, there are some cases where
-exceptions won't be automatically detected: any `Future` constructor,
-`Completer.completeError`, `Stream.addError`, and libraries that use these such
-as `dart:io` and `dart:async`. For these, all you need to do is wrap the Future
-or Stream in a call to `Chain.track` and the errors will be tracked correctly.
-
 [Zone]: https://api.dartlang.org/apidocs/channels/stable/#dart-async.Zone