Revert "Remove Expect from core library."

tests/language/nested_if_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.
 // Dart2Js had problems with nested ifs inside loops.
 
-import "package:expect/expect.dart";
-
 foo(x, a) {
   /*---------------------------------------------*/
   /*---------------------------------------------*/
   /*---------------------------------------------*/
   /*---------------------------------------------*/
   /*---------------------------------------------*/
   /*---------------------------------------------*/
   /*------- Avoid inlining ----------------------*/
   /*---------------------------------------------*/
   /*---------------------------------------------*/
   /*---------------------------------------------*/
   /*---------------------------------------------*/
   /*---------------------------------------------*/
   /*---------------------------------------------*/
   for (int i = 0; i < 10; i++) {
     if (x) {
       if (!x) a = [];
       a.add(3);
     }
   }
   return a;
 }
 
 main() {
   var a = foo(true, []);
   Expect.equals(10, a.length);
   Expect.equals(3, a[0]);
 }