Revert "Remove Expect from core library."

tests/language/prefix101_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.
 
 // Test to check if we are able to import multiple libraries using the
 // same prefix.
 
 library Prefix101Test.dart;
 
-import "package:expect/expect.dart";
 import "library10.dart" as lib101;
 import "library11.dart" as lib101;
 
 class Prefix101Test {
   static Test1() {
     var result = 0;
     var obj = new lib101.Library10(1);
     result = obj.fld;
     Expect.equals(1, result);
     result += obj.func();
@@ skipping 24 matching lines @@
     Expect.equals(200, lib101.top_level_func11());
   }
 }
 
 main() {
   Prefix101Test.Test1();
   Prefix101Test.Test2();
   Prefix101Test.Test3();
   Prefix101Test.Test4();
 }