Revert "Remove Expect from core library."

tests/language/cast_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.
 // Dart test program for constructors and initializers.
 
-import "package:expect/expect.dart";
-
 // Test 'expression as Type' casts.
 
 class C {
   final int foo = 42;
 }
 
 class D extends C {
   final int bar = 37;
 }
 
@@ skipping 49 matching lines @@
   (os as String).length;
   (os as dynamic).length;
   (oi as String).length;  /// 13: runtime error
   (os as List).length;  /// 14: runtime error
 
   (oi as int) + 2;
   (oi as List).length;  /// 15: runtime error
 }