Revert "Remove Expect from core library."

tests/language/mixin_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.
 
-import "package:expect/expect.dart";
-
 class S {
   foo() => "S-foo";
   baz() => "S-baz";
 }
 
 class M1 {
   static m1bar() => "M1-bar";
   bar() => m1bar();
 }
 
@@ skipping 29 matching lines @@
   Expect.equals("M1-bar", e.bar());
   Expect.equals("M2-baz", e.baz());
   Expect.equals("M2-fez", e.fez());
 
   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());
 }