Revert "Remove Expect from core library."

tests/compiler/dart2js_extra/is_operator_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.
 
-import "package:expect/expect.dart";
-
 class A {
 }
 
 class B extends A {
 }
 
 main() {
   var a = new A();
   var b = new B();
 
@@ skipping 12 matching lines @@
   Expect.equals("true", (b is A).toString());
   Expect.equals("true", (b is Object).toString());
   Expect.equals("true", (b is B).toString());
 
   var c = new A();
   Expect.isTrue(c is Object);
 
   c = new A();
   Expect.equals("true", (c is Object).toString());
 }