Revert "Remove Expect from core library."

tests/corelib/hash_set_test.dart

 // Copyright (c) 2013, 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.
 
 // Tests of hash set behavior, with focus in iteration and concurrent
 // modification errors.
 
 library hash_map2_test;
-import "package:expect/expect.dart";
 import 'dart:collection';
 
 testSet(Set newSet(), Set newSetFrom(Set from)) {
   Set gen(int from, int to) =>
       new Set.from(new Iterable.generate(to - from, (n) => n + from));
 
   bool odd(int n) => (n & 1) == 1;
   bool even(int n) => (n & 1) == 0;
 
   {  // Test growing to largish capacity.
@@ skipping 178 matching lines @@
 
 class BadHashCode {
   static int idCounter = 0;
   final int id;
   BadHashCode() : id = idCounter++;
   int get hashCode => 42;
   // operator == is identity.
   // Can't make a bad compareTo that isn't invalid.
   int compareTo(BadHashCode other) => id - other.id;
 }