fix MapKeySet.lookup to be a valid override in strong mode

lib/src/wrappers.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.
 
 import "dart:collection";
 import "dart:math" as math;
 
 import "typed_wrappers.dart";
 import "unmodifiable_wrappers.dart";
 
@@ skipping 431 matching lines @@
   ///
   /// That is, the returned set contains all the elements of this [Set] that are
   /// also elements of [other] according to `other.contains`.
   ///
   /// Note that the returned set will use the default equality operation, which
   /// may be different than the equality operation [this] uses.
   Set<E> intersection(Set<Object> other) => where(other.contains).toSet();
 
   /// Throws an [UnsupportedError] since there's no corresponding method for
   /// [Map]s.
-  E lookup(E element) =>
+  E lookup(Object element) =>

[Jennifer Messerly, 2017/07/19 23:47:36]

after this change, this is now a valid implementation of Set.lookup:
https://api.dartlang.org/stable/1.24.2/dart-core/Set/lookup.html

       throw new UnsupportedError("MapKeySet doesn't support lookup().");
 
   /// Returns a new set which contains all the elements of [this] and [other].
   ///
   /// That is, the returned set contains all the elements of this [Set] and all
   /// the elements of [other].
   ///
   /// Note that the returned set will use the default equality operation, which
   /// may be different than the equality operation [this] uses.
   Set<E> union(Set<E> other) => toSet()..addAll(other);
@@ skipping 133 matching lines @@
 
   /// Returns a new set which contains all the elements of [this] and [other].
   ///
   /// That is, the returned set contains all the elements of this [Set] and all
   /// the elements of [other].
   ///
   /// Note that the returned set will use the default equality operation, which
   /// may be different than the equality operation [this] uses.
   Set<V> union(Set<V> other) => toSet()..addAll(other);
 }