Revert "Remove Expect from core library."

tests/language/is_interfaces_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 {
 }
 
 class C implements B {
 }
 
 int inscrutable(int x) => x == 0 ? 0 : x | inscrutable(x & (x - 1));
 
 main() {
   var things = [new A(), new B(), new C()];
 
   var a = things[inscrutable(0)];
   Expect.isTrue(a is A);
   Expect.isFalse(a is B);
   Expect.isFalse(a is C);
 
   var b = things[inscrutable(1)];
   Expect.isTrue(b is A);
   Expect.isTrue(b is B);
   Expect.isFalse(b is C);
 
   var c = things[inscrutable(2)];
   Expect.isTrue(c is A);
   Expect.isTrue(c is B);
   Expect.isTrue(c is C);
 }