Revert "Remove Expect from core library."

tests/language/mixin_type_parameters_mixin_extends_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.
 
-import "package:expect/expect.dart";
-
 class M<T> {
   bool matches(o) {
     bool isChecked = checkUsingIs(o);
     if (checkedMode) {
       Expect.equals(isChecked, checkUsingCheckedMode(o));
     }
     return isChecked;
   }
 
   bool checkUsingIs(o) {
@@ skipping 102 matching lines @@
   Expect.isFalse(c2_String.matches("hello"));
 
   var c3 = new C3();
   Expect.isTrue(c3 is M);
   Expect.isFalse(c3 is M<int>);
   Expect.isTrue(c3 is M<String>);
   Expect.isFalse(c3.matches(c2));
   Expect.isFalse(c3.matches(42));
   Expect.isTrue(c3.matches("hello"));
 }