Revert "Remove Expect from core library."

tests/language/mint_arithmetic_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.
 
-import "package:expect/expect.dart";
-
 // Test arithmetic on 64-bit integers.
 
 test_and_1() {
   try {  // Avoid optimizing this function.
     f(a, b) {
       var s = b;
       var t = a & s;
       return t == b;
     }
     var x = 0xffffffff;
@@ skipping 60 matching lines @@
 
 main() {
   for (var i = 0; i < 5; i++) {
     test_and_1();
     test_and_2();
     test_xor_1();
     test_or_1();
     test_mint_double_op();
   }
 }