Revert "Remove Expect from core library."

pkg/fixnum/test/int_32_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.
 
 library int32test;
-import "package:expect/expect.dart";
 import 'package:fixnum/fixnum.dart';
 
 void main() {
   Expect.equals("0", new int32.fromInt(0).toString());
   Expect.equals("1", new int32.fromInt(1).toString());
   Expect.equals("-1", new int32.fromInt(-1).toString());
   Expect.equals("1000", new int32.fromInt(1000).toString());
   Expect.equals("-1000", new int32.fromInt(-1000).toString());
   Expect.equals("2147483647", new int32.fromInt(2147483647).toString());
   Expect.equals("-2147483648", new int32.fromInt(2147483648).toString());
@@ skipping 133 matching lines @@
   }
 
   try {
     new int32.fromInt(17) < null;
     Expect.fail("x >= null should throw ArgumentError");
   } on ArgumentError catch (e) {
   }
 
   Expect.isFalse(new int32.fromInt(17) == null);
 }