Revert "Remove Expect from core library."

tests/compiler/dart2js_extra/regression_2913_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() {
   var a = 10;
   var b = 11;
 
   for (var i = 0; i < 1; i++) {
     print('--------');
     print('a $a');
     print('b $b');
     var t = 1;
     if (i < 20) {
       t = 2;
     }
 
     b = a;
     a = t;
   }
   Expect.equals(2, a);
   Expect.equals(10, b);
 }