Allow Object when doing lookups.

tests/corelib/set_remove_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 { const A(); }
+class B extends A { const B(); }
+
+main() {
+  var set1 = new Set<B>();
+  set1.add(const B());
+  var set2 = new Set<B>();
+  var list = <B>[ const B() ];
+  var set3 = list.toSet();
+
+  var sets = [set1, set2, set3];
+  for (var setToTest in sets) {
+    // Test that the set accepts a value that is not of the same type
+    //   Set<B>.remove(A)
+    Expect.isFalse(setToTest.remove(new A()));
+  }
+}