Make the analyzer happy about collections.

sdk/lib/collection/hash_set.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.
 
 part of dart.collection;
 
 /** Common parts of [HashSet] and [LinkedHashSet] implementations. */
 abstract class _HashSetBase<E> extends Collection<E> implements Set<E> {
   // Set.
   bool isSubsetOf(Collection<E> other) {
     // Deprecated, and using old signature.
     Set otherSet;
     if (other is Set) {
       otherSet = other;
     } else {
       otherSet = other.toSet();
     }
     return otherSet.containsAll(this);
   }
 
   bool containsAll(Iterable<E> other) {
     for (E object in other) {
       if (!this.contains(object)) return false;
     }
     return true;
   }
 
+  /** Create a new Set of the same type as this. */
+  Set _newSet();
+
   Set<E> intersection(Set<E> other) {
     Set<E> result = _newSet();
     if (other.length < this.length) {
       for (E element in other) {
         if (this.contains(element)) result.add(element);
       }
     } else {
       for (E element in this) {
         if (other.contains(element)) result.add(element);
       }
@@ skipping 53 matching lines @@
 
   external void removeWhere(bool test(E element));
 
   external void retainWhere(bool test(E element));
 
   external void clear();
 
   // Set.
   Set<E> _newSet() => new HashSet<E>();
 }