Improve document for HashSet.

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 SetBase<E> {
 
   // The following two methods override the ones in SetBase.
@@ skipping 90 matching lines @@
    * Effectively a shorthand for:
    *
    *     new HashSet<E>(equals: identical,
    *                    hashCode: identityHashCode)
    */
   external factory HashSet.identity();
 
   /**
    * Create a hash set containing all [elements].
    *
-   * Creates a hash set as by `new HashSet<E>()` and adds each element of
-   * `elements` to this set in the order they are iterated.
+   * Creates a hash set as by `new HashSet<E>()` and adds all given [elements]
+   * to the set. The elements are added in order. If [elements] contains
+   * two entries that are equal, but not identical, then the first one is
+   * the one in the resulting set.

[Lasse Reichstein Nielsen, 2016/12/13 14:01:33]

Good idea.

[floitsch, 2016/12/13 14:08:31]

Acknowledged.

+   *
+   * The iteration order of the resulting set is undetermined and depends on

[Lasse Reichstein Nielsen, 2016/12/13 14:01:33]

I don't think we have used "undetermined" anywhere before. Maybe "implementation
dependent" or just "not specified' or "not guaranteed".

[floitsch, 2016/12/13 14:08:31]

not specified.

+   * the hashcodes of the provided elements. However, the order is stable:
+   * multiple iterations over the same set produce the same order, as long as
+   * the set is not modified.

[Lasse Reichstein Nielsen, 2016/12/13 14:01:33]

I'm not sure we need to repeat what unordered set means in every constructor.
That said, it's not really *repeating* since we haven't spelled it out anywhere.
How about moving this paragraph to the class description instead of here?

[floitsch, 2016/12/13 14:08:31]

Done.

    *
    * All the [elements] should be assignable to [E].
    * The `elements` iterable itself may have any element type, so this
    * constructor can be used to down-cast a `Set`, for example as:
    *
    *     Set<SuperType> superSet = ...;
    *     Set<SubType> subSet =
    *         new HashSet<SubType>.from(superSet.where((e) => e is SubType));
    */
   factory HashSet.from(Iterable elements) {
     HashSet<E> result = new HashSet<E>();
     for (final e in elements) {
       E element = e as Object/*=E*/;
       result.add(element);
     }
     return result;
   }
 
   /**
    * Provides an iterator that iterates over the elements of this set.
    *
    * The order of iteration is unspecified,
    * but consistent between changes to the set.
    */
   Iterator<E> get iterator;
 }