Revert "Remove Expect from core library."

tests/language/mixin_illegal_super_use_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 {
 }
 
 class P0 {
   foo() {
     super.toString();    /// 01: compile-time error
     super.foo();         /// 02: compile-time error
     super.bar = 100;     /// 03: compile-time error
 
     void inner() {
@@ skipping 45 matching lines @@
 typedef E = Object with M, P1;
 typedef F = Object with P2, M;
 
 main() {
   var p1 = new P1();
   var p2 = new P2();
   Expect.equals(87, p1.bar());
   p1.test();
   Expect.equals(99, p2.baz());
 }