Revert "Remove Expect from core library."

tests/corelib/num_clamp_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.
 // Test num.clamp.
 
-import "package:expect/expect.dart";
-
 testIntClamp() {
   Expect.equals(2, 2.clamp(1, 3));
   Expect.equals(1, 0.clamp(1, 3));
   Expect.equals(3, 4.clamp(1, 3));
   Expect.equals(-2, (-2).clamp(-3, -1));
   Expect.equals(-1, 0.clamp(-3, -1));
   Expect.equals(-3, (-4).clamp(-3, -1));
   Expect.equals(0, 1.clamp(0, 0));
   Expect.equals(0, (-1).clamp(0, 0));
   Expect.equals(0, 0.clamp(0, 0));
@@ skipping 53 matching lines @@
   Expect.throws(() => 0.0.clamp(double.NAN, double.INFINITY),
                 (e) => e is ArgumentError);
 }
 
 main() {
   testIntClamp();
   testDoubleClamp();
   testDoubleClampInt();
   testDoubleClampExtremes();
 }