Revert "Remove Expect from core library."

tests/corelib/list_fixed_test.dart

 // Copyright (c) 2012, 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";
-
 main() {
   var a;
 
   a = new List(42);
   Expect.equals(42, a.length);
   Expect.throws(() => a.add(499), (e) => e is UnsupportedError);
   Expect.equals(42, a.length);
   for (int i = 0; i < 42; i++) {
     Expect.equals(null, a[i]);
   }
   Expect.throws(() => a.clear(), (e) => e is UnsupportedError);
   Expect.equals(42, a.length);
 
   a = new List.filled(42, -2);
   Expect.equals(42, a.length);
   Expect.throws(() => a.add(499), (e) => e is UnsupportedError);
   Expect.equals(42, a.length);
   for (int i = 0; i < 42; i++) {
     Expect.equals(-2, a[i]);
   }
   Expect.throws(() => a.clear(), (e) => e is UnsupportedError);
   Expect.equals(42, a.length);
 }