Improved the way we generate mismatch descriptions.

tests/lib/unittest/instance_test.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.
 
-
 #library('instance_test');
 #import('../../../lib/unittest/unittest.dart');
 #source('test_utils.dart');
 
 doesThrow() { throw 'X'; }
 
 main() {
   initUtils();
 
   group('Type Matchers', () {
     test('isInstanceOf', () {
       shouldFail(0, new isInstanceOf<String>('String'),
         "Expected: an instance of String but: was <0>.");
       shouldPass('cow', new isInstanceOf<String>('String'));
     });
 
     test('throwsA', () {
       shouldPass(doesThrow, throwsA(equals('X')));
       shouldFail(doesThrow, throwsA(equals('Y')),
         "Expected: throws an exception which matches 'Y' "
-        "but:  no exception or exception does not match 'Y'.");
+        "but:  exception 'X' does not match 'Y'.");
     });
   });
 }