Remove deprecated Expect from the libraries.

tests/language/mixin_field_initializer_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 program for testing throw statement
 
+import 'package:expect/expect.dart';
+
 class S {
   var s1 = good_stuff();
   static good_stuff() => "Speyburn";
 }
 
 good_stuff() => "Glenfiddich";
 
 class M {
   var m1 = good_stuff();
   static good_stuff() => "Macallen";
 }
 
 class A extends S with M {
   static good_stuff() => "Ardberg";
 }
 
 main() {
   var a = new A();
   Expect.equals("Macallen", a.m1);
   Expect.equals("Speyburn", a.s1);
 
   var m = new M();
   Expect.equals("Macallen", m.m1);
 }