Revert "Remove Expect from core library."

tests/language/number_identity_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 program for testing params.
 
-import "package:expect/expect.dart";
-
 main() {
   for (int i = 0; i < 1000; i++) testNumberIdentity();
 }
 
 
 testNumberIdentity () {
   const int smi = 8;
   const int mint = 9223372036854775806;
   const int bigint = 22107138293752210713829375;
   const double dbl = 8.0;
@@ skipping 17 matching lines @@
   Expect.isFalse(identical(b, smi));
   Expect.isFalse(identical(b, mint));
   Expect.isFalse(identical(b, dbl));
 
   a = dbl;
   b = a + 0.0;
   Expect.isTrue(identical(a, b));
   Expect.isFalse(identical(b, mint));
   Expect.isFalse(identical(b, bigint));
 }