Add error-intercept for Completer.completeError and StreamController.addError.

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;
 
 _invokeErrorHandler(Function errorHandler,
                     Object error, StackTrace stackTrace) {
   if (errorHandler is ZoneBinaryCallback) {
     return errorHandler(error, stackTrace);
   } else {
     return errorHandler(error);
   }
 }
 
 Function _registerErrorHandler(Function errorHandler, Zone zone) {
   if (errorHandler is ZoneBinaryCallback) {
     return zone.registerBinaryCallback(errorHandler);
   } else {
     return zone.registerUnaryCallback(errorHandler);
   }
 }
 
-class _AsyncError implements Error {
-  final error;
-  final StackTrace stackTrace;
-
-  _AsyncError(this.error, this.stackTrace);
-}
-
-class _UncaughtAsyncError extends _AsyncError {
+class _UncaughtAsyncError extends AsyncError {
   _UncaughtAsyncError(error, StackTrace stackTrace)
       : super(error, _getBestStackTrace(error, stackTrace));
 
   static StackTrace _getBestStackTrace(error, StackTrace stackTrace) {
     if (stackTrace != null) return stackTrace;
     if (error is Error) {
       return error.stackTrace;
     }
     return null;
   }
 
   String toString() {
     String result = "Uncaught Error: ${error}";
 
     if (stackTrace != null) {
       result += "\nStack Trace:\n$stackTrace";
     }
     return result;
   }
 }