VM: Remove explicit field initialization with null from library classes.

runtime/lib/collection_patch.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.
 
 patch class HashMap<K, V> {
   /* patch */ factory HashMap({ bool equals(K key1, K key2),
                                 int hashCode(K key),
                                 bool isValidKey(potentialKey) }) {
     if (isValidKey == null) {
       if (hashCode == null) {
@@ skipping 758 matching lines @@
     _HashSetEntry result = next;
     next = null;
     return result;
   }
 }
 
 class _HashSetIterator<E> implements Iterator<E> {
   final _HashSet _set;
   final int _modificationCount;
   int _index = 0;
-  _HashSetEntry _next = null;
-  E _current = null;
+  _HashSetEntry _next;
+  E _current;
 
   _HashSetIterator(_HashSet hashSet)
       : _set = hashSet, _modificationCount = hashSet._modificationCount;
 
   bool moveNext() {
     if (_modificationCount != _set._modificationCount) {
       throw new ConcurrentModificationError(_set);
     }
     if (_next != null) {
       _current = _next.key;
@@ skipping 53 matching lines @@
   int get length => _map.length;
 }
 
 abstract class _LinkedHashMapIterator<T> implements Iterator<T> {
   final LinkedHashMap _map;
   var _next;
   T _current;
   int _modificationCount;
   _LinkedHashMapIterator(LinkedHashMap map)
       : _map = map,
-        _current = null,
         _next = map._nextEntry,
         _modificationCount = map._modificationCount;
 
   bool moveNext() {
     if (_modificationCount != _map._modificationCount) {
       throw new ConcurrentModificationError(_map);
     }
     if (identical(_map, _next)) {
       _current = null;
       return false;
@@ skipping 376 matching lines @@
       return false;
     }
     _LinkedHashSetEntry entry = _next;
     _current = entry.key;
     _next = entry._nextEntry;
     return true;
   }
 
   E get current => _current;
 }