Revert "Remove Expect from core library."

tests/language/setter2_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.
 // Dart test program for testing use of 'this' in an instance method.
 
-import "package:expect/expect.dart";
-
 class Nested {
   Nested(int val) : a = val { }
   int a;
   int foo(int i) {
     return i;
   }
 }
 
 class Second {
   int a;
@@ skipping 17 matching lines @@
     Expect.equals(10, Second.obj.a);
     Expect.equals(10, Second.obj.foo(10));
     obj.bar(20);
     Expect.equals(20, Second.obj.a);
   }
 }
 
 main() {
   Setter2Test.testMain();
 }