Revert "Remove Expect from core library."

tests/corelib/string_codeunits_test.dart

 // Copyright (c) 2011, 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(String s) {
     Iterable<int> units = s.codeUnits;
     List<int> expectedUnits = <int>[];
     for (int i = 0; i < s.length; i++) {
       expectedUnits.add(s.codeUnitAt(i));
     }
 
     Expect.equals(s.length, units.length);
     for (int i = 0; i < s.length; i++) {
@@ skipping 51 matching lines @@
   test(string);
 
   // Reading each unit of a surrogate pair works.
   var r = "\u{10000}".codeUnits;
   var it = r.iterator;
   Expect.isTrue(it.moveNext());
   Expect.equals(0xD800, it.current);
   Expect.isTrue(it.moveNext());
   Expect.equals(0xDC00, it.current);
 }