Revert "Remove Expect from core library."

tests/corelib/iterable_min_max_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.
 
 library iterable_min_max_test;
 
-import "package:expect/expect.dart";
 import "dart:collection";
 
 class C {
   final x;
   const C(this.x);
   int get hashCode => x.hashCode;
   bool operator==(var other) => other is C && x == other.x;
 }
 
 const inf = double.INFINITY;
@@ skipping 54 matching lines @@
   Expect.equals(const C(10), IterableMixinWorkaround.max(new Set.from(cList), compareC));
 
   bool checkedMode = false;
   assert(checkedMode = true);
   Expect.throws(cList.min, (e) => checkedMode ? e is TypeError
                                               : e is NoSuchMethodError);
   Expect.throws(cList.max, (e) => checkedMode ? e is TypeError
                                               : e is NoSuchMethodError);
 }