Revert "Remove Expect from core library."

tests/language/fast_method_extraction_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 that fast method extraction returns correct closure.
 
-import "package:expect/expect.dart";
-
 class A {
   var f;
   A(this.f);
   foo() => 40 + f;
 }
 
 class B {
   var f;
   B(this.f);
   foo() => -40 - f;
@@ skipping 79 matching lines @@
   var chaB = new ChaB("magicB");
   for (var i = 0; i < 10000; i++) {
     Expect.equals("B", cha(chaB));
   }
 
   for (var i = 0; i < 10000; i++) {
     extractFromNull();
   }
 }