Revert "Remove Expect from core library."

tests/language/mixin_field_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 {
   var foo = "S-foo";
 }
 
 class M1 {
   final bar = "M1-bar";
 }
 
 class M2 {
   var baz = "M2-baz";
@@ skipping 94 matching lines @@
   Expect.throws(() => c.fez = 0, (error) => error is NoSuchMethodError);
   Expect.throws(() => d.fez = 0, (error) => error is NoSuchMethodError);
   Expect.throws(() => e.fez = 0, (error) => error is NoSuchMethodError);
 
   f.fez = "F-fez-f";
   Expect.throws(() => c.fez, (error) => error is NoSuchMethodError);
   Expect.throws(() => d.fez, (error) => error is NoSuchMethodError);
   Expect.throws(() => e.fez, (error) => error is NoSuchMethodError);
   Expect.equals("F-fez-f", f.fez);
 }