Revert "Remove Expect from core library."

tests/corelib/collection_removes_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";
-
 testRemove(Collection base) {
   int length = base.length;
   for (int i = 0; i < length; i++) {
     Expect.isFalse(base.isEmpty);
     base.remove(base.first);
   }
   Expect.isTrue(base.isEmpty);
 }
 
 testRemoveAll(Collection base, Iterable removes) {
@@ skipping 86 matching lines @@
       testRemoveAll(base.toSet(), delta);
       testRemoveAll(base.toSet(), deltaSet);
       testRetainAll(base.toSet(), delta);
       testRetainAll(base.toSet(), deltaSet);
       testRemoveMatching(base.toSet(), deltaSet.contains);
       testRetainMatching(base.toSet(), (e) => !deltaSet.contains(e));
     }
   }
 }