Re-define ApplicationException.toString() in pub.

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

Index: sdk/lib/_internal/pub/lib/src/exceptions.dart
diff --git a/sdk/lib/_internal/pub/lib/src/exceptions.dart b/sdk/lib/_internal/pub/lib/src/exceptions.dart
index e7f4a28f02a571484d768a05c6ac827496f4e255..e67ef64dab3e2af57eff1b448ffece918d9d1bb7 100644
--- a/sdk/lib/_internal/pub/lib/src/exceptions.dart
+++ b/sdk/lib/_internal/pub/lib/src/exceptions.dart
@@ -4,7 +4,6 @@
 
 library pub.exceptions;
 
-import 'dart:async';
 import 'dart:io';
 import 'dart:isolate';
 
@@ -23,6 +22,8 @@ class ApplicationException implements Exception {
   final String message;
 
   ApplicationException(this.message);
+
+  String toString() => message;
 }
 
 /// A class for exceptions that wrap other exceptions.