Fix build for throwsStateError: remove duplicate definitions in other tests.

utils/tests/pub/error_group_test.dart

 // Copyright (c) 2013, 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 error_group_test;
 
 import 'dart:async';
 
 import '../../../pkg/unittest/lib/unittest.dart';
 import '../../pub/error_group.dart';
@@ skipping 433 matching lines @@
       expect(stream.toList(), completion(equals(['value1', 'value2'])));
       controller..add('value1')..add('value2')..close();
 
       expect(stream.toList().then((_) {
         // shouldn't cause a top-level exception
         completer.completeError(new FormatException());
       }), completes);
     });
   });
 }
-
-// TODO(nweiz): remove this once it's built in to unittest (issue 7922).
-/// A matcher for StateErrors.
-const isStateError = const _StateError();
-
-/// A matcher for functions that throw StateError.
-const Matcher throwsStateError =
-    const Throws(isStateError);
-
-class _StateError extends TypeMatcher {
-  const _StateError() : super("StateError");
-  bool matches(item, MatchState matchState) => item is StateError;
-}