Fix all strong-mode warnings.

lib/src/utils/multiset.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.
 
 library barback.utils.multiset;
 
 import 'dart:collection';
 
 /// A set of objects where each object can appear multiple times.
 ///
 /// Like a set, this has amortized O(1) insertion, removal, and
 /// existence-checking of elements. Counting the number of copies of an element
 /// in the set is also amortized O(1).
 ///
 /// Distinct elements retain insertion order. Additional copies of an element
 /// beyond the first are grouped with the original element.
 ///
 /// If multiple equal elements are added, only the first actual object is
 /// retained.
 class Multiset<E> extends IterableBase<E> {
   /// A map from each element in the set to the number of copies of that element
   /// in the set.
   final _map = new Map<E, int>();
 
   Iterator<E> get iterator {
-    return _map.keys.expand((element) {
-      return new Iterable.generate(_map[element], (_) => element);
-    }).iterator;
+    return _map.keys
+        .expand((element) =>
+            new Iterable<E>.generate(_map[element], (_) => element))
+        .iterator;
   }
 
   Multiset()
       : super();
 
   /// Creates a multi-set and initializes it using the contents of [other].
   Multiset.from(Iterable<E> other)
       : super() {
     other.forEach(add);
   }
@@ skipping 10 matching lines @@
   /// copies remain.
   bool remove(E value) {
     if (!_map.containsKey(value)) return false;
 
     _map[value] -= 1;
     if (_map[value] == 0) _map.remove(value);
     return true;
   }
 
   /// Returns whether [value] is in the set.
-  bool contains(E value) => _map.containsKey(value);
+  bool contains(Object value) => _map.containsKey(value);
 
   /// Returns the number of copies of [value] in the set.
   int count(E value) => _map.containsKey(value) ? _map[value] : 0;
 }