Re-define ApplicationException.toString() in pub.

sdk/lib/_internal/pub/lib/src/exceptions.dart

 // Copyright (c) 2014, 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.
 
 library pub.exceptions;
 
-import 'dart:async';
 import 'dart:io';
 import 'dart:isolate';
 
 import "package:analyzer/analyzer.dart";
 import "package:http/http.dart" as http;
 import "package:stack_trace/stack_trace.dart";
 import "package:yaml/yaml.dart";
 
 import '../../asset/dart/serialize.dart';
 
 /// An exception class for exceptions that are intended to be seen by the user.
 ///
 /// These exceptions won't have any debugging information printed when they're
 /// thrown.
 class ApplicationException implements Exception {
   final String message;
 
   ApplicationException(this.message);
+
+  String toString() => message;
 }
 
 /// A class for exceptions that wrap other exceptions.
 class WrappedException extends ApplicationException {
   /// The underlying exception that [this] is wrapping, if any.
   final innerError;
 
   /// The stack chain for [innerError] if it exists.
   final Chain innerChain;
 
@@ skipping 60 matching lines @@
   // TODO(nweiz): unify this list with _userFacingExceptions when issue 5897 is
   // fixed.
   return error is ApplicationException ||
     error is AnalyzerError ||
     error is AnalyzerErrorGroup ||
     error is IsolateSpawnException ||
     error is IOException ||
     error is http.ClientException ||
     error is YamlException;
 }