Remove deprecated Expect from the libraries.

tests/language/private_mixin2_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.
 
 // Dart test for testing access to private fields on mixins.
 
 library private_mixin2;
 
+import 'package:expect/expect.dart';
 import 'private_other_mixin2.lib';
 
 void main() {
   var c1 = new C1();
   Expect.throws(() => c1._field, (e) => e is NoSuchMethodError);
   Expect.throws(() => c1.field, (e) => e is NoSuchMethodError);
 
   var c2 = new C2();
   Expect.throws(() => c2._field, (e) => e is NoSuchMethodError);
   Expect.equals(42, c2.field);
 }