Revert "Remove Expect from core library."

tests/corelib/integer_to_radix_string_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";
-
 main() {
   // Test that we accept radix 2 to 36 and that we use lower-case
   // letters.
   var expected = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
                   'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j',
                   'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't',
                   'u', 'v', 'w', 'x', 'y', 'z'];
   for (var radix = 2; radix < 37; radix++) {
     for (var i = 0; i < radix; i++) {
       Expect.equals(expected[i], i.toRadixString(radix));
     }
   }
 
   var illegalRadices = [ -1, 0, 1, 37 ];
   for (var radix in illegalRadices) {
     try {
       42.toRadixString(radix);
       Expect.fail("Exception expected");
     } on ArgumentError catch (e) {
       // Nothing to do.
     }
   }
 }