Revert "Remove Expect from core library."

tests/language/type_propagation_in_for_update_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.
 // Check that phi type computation in the Dart2Js compiler does the
 // correct thing.
 
-import "package:expect/expect.dart";
-
 bar() => 'foo';
 
 main() {
   Expect.throws(foo1);
   Expect.throws(foo2);
 }
 
 foo1() {
   var a = bar();
   for (;; a = 1 + a) {
     if (a != 'foo') return;
   }
 }
 
 foo2() {
   var a = bar();
   for (;; a = 1 + a) {
     if (a != 'foo') break;
   }
 }