Revert "Remove Expect from core library."

tests/language/const_objects_are_immutable_test.dart

 // Copyright (c) 2011, 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.
 // Check that const objects (including literals) are immutable.
 
-import "package:expect/expect.dart";
-
 class A {
   const A(this.x, this.y);
   final num x, y;
 }
 
 main() {
   var list = const [1, 2];
   Expect.throws(() => list[0] = 3);
   Expect.equals(1, list[0]);
 
   var m = const { 'foo': 499 };
   Expect.throws(() => m['foo'] = 42);
   Expect.equals(499, m['foo']);
 
   var a1 = const A(1, 2);
   Expect.throws(() => a1.x = 499);
   Expect.equals(1, a1.x);
 
   A a2 = const A(1, 2);
   Expect.throws(() => a2.x = 499);
   Expect.equals(1, a2.x);
 }