Fix inconsistent APIs

lib/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.
 
 /**
  * Delegating wrappers for [Iterable], [List], [Set], [Queue] and [Map].
  *
  * Also adds unmodifiable views for `Set` and `Map`, and a fixed length
  * view for `List`. The unmodifable list view from `dart:collection` is exported
  * as well, just for completeness.
@@ skipping 216 matching lines @@
   void clear() {
     _setBase.clear();
   }
 
   bool containsAll(Iterable<Object> other) => _setBase.containsAll(other);
 
   Set<E> difference(Set<E> other) => _setBase.difference(other);
 
   Set<E> intersection(Set<Object> other) => _setBase.intersection(other);
 
-  E lookup(E element) => _setBase.lookup(element);
+  E lookup(Object element) => _setBase.lookup(element);
 
   bool remove(Object value) => _setBase.remove(value);
 
   void removeAll(Iterable<Object> elements) {
     _setBase.removeAll(elements);
   }
 
   void removeWhere(bool test(E element)) {
     _setBase.removeWhere(test);
   }
@@ skipping 263 matching lines @@
    * Returns a new set which is the intersection between [this] and [other].
    *
    * 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<V> intersection(Set<Object> other) => where(other.contains).toSet();
 
-  V lookup(V element) => _baseMap[_keyForValue(element)];
+  V lookup(Object element) => _baseMap[_keyForValue(element)];
 
   bool remove(Object value) {
     if (value != null && value is! V) return false;
     var key = _keyForValue(value);
     if (!_baseMap.containsKey(key)) return false;
     _baseMap.remove(key);
     return true;
   }
 
   void removeAll(Iterable<Object> elements) => elements.forEach(remove);
@@ skipping 29 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);
 }