Revert "Remove Expect from core library."

tests/language/operator4_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.
 
-import "package:expect/expect.dart";
-
 class A {
   operator <(other) => 1;
 }
 
 // This triggered a bug in Dart2Js: relational operators were not correctly
 // boolified.
 foo(a) {
   try {
     if (a < a) {
       return "bad";
     } else {
       return 499;
     }
   } on TypeError catch (e) {
     return 499;
   }
 }
 
 main() {
   Expect.equals(499, foo(new A()));
 }