Revert "Convert unittest to use "package:" imports."

pkg/unittest/test/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 unittestTests;
-import 'package:unittest/unittest.dart';
+import '../../../pkg/unittest/lib/unittest.dart';
 
 part '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:  exception 'X' does not match 'Y'.");
     });
   });
 }