Revert "Remove Expect from core library."

tests/standalone/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.
 //
 // Tests 'identical' for cases that not supported in dart2js (bigint,
 // disambiguation int/double).
 
-import "package:expect/expect.dart";
-
 main() {
   for (int i = 0; i < 1000; i++) testNumberIdentity();
 }
 
 
 testNumberIdentity () {
   const int smi = 8;
   const int bigint = 22107138293752210713829375;
   const double dbl = 8.0;
   // No int/double differences in dart2js.
   var a = smi + 0;
   Expect.isFalse(identical(a, dbl));
   a = dbl + 0.0;
   Expect.isFalse(identical(a, smi));
 
   a = bigint;
   var b = a + 0;
   Expect.isTrue(identical(a, b));
   b = a + 1;
   Expect.isFalse(identical(a, b));  // Fails with dart2js.
 }