Add SetBase/SetMixin.

sdk/lib/collection/hash_set.dart

Index: sdk/lib/collection/hash_set.dart
diff --git a/sdk/lib/collection/hash_set.dart b/sdk/lib/collection/hash_set.dart
index ff95b4306541a488d0bb5daf90f5df34278f0aa6..359c1a4c434fa6673a0d250fc7fb4b59f71c6ce1 100644
--- a/sdk/lib/collection/hash_set.dart
+++ b/sdk/lib/collection/hash_set.dart
@@ -5,59 +5,7 @@
 part of dart.collection;
 
 /** Common parts of [HashSet] and [LinkedHashSet] implementations. */
-abstract class _HashSetBase<E> extends IterableBase<E> implements Set<E> {
-
-  // Set.
-  bool containsAll(Iterable<Object> other) {
-    for (Object object in other) {
-      if (!this.contains(object)) return false;
-    }
-    return true;
-  }
-
-  /** Create a new Set of the same type as this. */
-  HashSet<E> _newSet();
-
-  Set<E> intersection(Set<Object> other) {
-    Set<E> result = _newSet();
-    for (var element in this) {
-      if (other.contains(element)) result.add(element);
-    }
-    return result;
-  }
-
-  Set<E> union(Set<E> other) {
-    return _newSet()..addAll(this)..addAll(other);
-  }
-
-  Set<E> difference(Set<E> other) {
-    HashSet<E> result = _newSet();
-    for (E element in this) {
-      if (!other.contains(element)) result.add(element);
-    }
-    return result;
-  }
-
-  void _retainAll(Iterable objectsToRetain, bool isValidKey(Object o)) {
-    // TODO(lrn): Consider optimizing table based versions by
-    // building a new table of the entries to retain.
-    Set retainSet = _newSet();
-    for (Object o in objectsToRetain) {
-      if (isValidKey(o)) {
-        retainSet.add(o);
-      }
-    }
-    retainWhere(retainSet.contains);
-  }
-
-  List<E> toList({bool growable: true}) {
-    List<E> result = growable ? (new List<E>()..length = this.length)
-                              : new List<E>(this.length);
-    int i = 0;
-    for (E element in this) result[i++] = element;
-    return result;
-  }
-
+abstract class _HashSetBase<E> extends SetBase<E> {
   Set<E> toSet() => _newSet()..addAll(this);
 
   String toString() => IterableMixinWorkaround.toStringIterable(this, '{', '}');