Revert "Remove Expect from core library."

tests/language/mixin_lib_extends_method_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 mixin_lib_extends_method_test;
 
-import "package:expect/expect.dart";
 import "mixin_lib_extends_method_lib.dart" as L;
 
 class S {
   foo() => "S-foo";
   baz() => "S-baz";
 }
 
 class C extends S with L.M1 { }
 class D extends S with L.M1, L.M2 { }
 class E extends S with L.M2, L.M1 { }
@@ skipping 24 matching lines @@
   Expect.equals("M2-fez", e.fez());
   Expect.equals("sugus", e.clo("su")("gus"));
 
   var f = new F();
   Expect.equals("S-foo", f.foo());
   Expect.equals("M1-bar", f.bar());
   Expect.equals("M2-baz", f.baz());
   Expect.equals("F-fez", f.fez());
   Expect.equals("sugus", f.clo("su")("gus"));
 }