Revert "Remove Expect from core library."

tests/language/default_init_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.
 
-import "package:expect/expect.dart";
-
 // Tests static and instance fields initialization.
 class DefaultInitTest {
   static testMain() {
     Expect.equals(0, A.a);
     Expect.equals(2, A.b);
     Expect.equals(null, A.c);
 
     A a1 = new A(42);
     Expect.equals(42, a1.d);
     Expect.equals(null, a1.e);
@@ skipping 40 matching lines @@
 }
 
 class D {
   const D(): this.z = 3;
   final int z;
 }
 
 main() {
   DefaultInitTest.testMain();
 }