Revert "Remove Expect from core library."

tests/corelib/compare_to_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.
 // Dart test for testing Math.min and Math.max.
 
-import "package:expect/expect.dart";
-
 negate(x) => -x;
 
 main() {
   // Test matrix:
   // -inf < -499.0 == -499 < -0.0 < 0.0 == 0 < 499.0 == 499 < +inf < -NaN, NaN.
   var inf = double.INFINITY;
   var nan = double.NAN;
   var mnan = negate(nan);
 
   Expect.equals(0, (-inf).compareTo(-inf));
@@ skipping 109 matching lines @@
   Expect.equals(1, nan.compareTo(-499));
   Expect.equals(1, nan.compareTo(-0.0));
   Expect.equals(1, nan.compareTo(0));
   Expect.equals(1, nan.compareTo(0.0));
   Expect.equals(1, nan.compareTo(499.0));
   Expect.equals(1, nan.compareTo(499));
   Expect.equals(1, nan.compareTo(inf));
   Expect.equals(0, nan.compareTo(nan));
   Expect.equals(0, nan.compareTo(mnan));
 }