Make dart:collection strong-mode clean.

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 103 matching lines @@
    * 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 (E e in elements) result.add(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;
 }