Make dart:collection strong-mode clean.

sdk/lib/collection/linked_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;
 
 /**
  * A [LinkedHashSet] is a hash-table based [Set] implementation.
  *
  * The `LinkedHashSet` also keep track of the order that elements were inserted
@@ skipping 86 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 = ...;
    *     Iterable<SuperType> tmp = superSet.where((e) => e is SubType);
    *     Set<SubType> subSet = new LinkedHashSet<SubType>.from(tmp);
    */
   factory LinkedHashSet.from(Iterable elements) {
     LinkedHashSet<E> result = new LinkedHashSet<E>();
-    for (final E element in elements) {
-      result.add(element);
+    for (final element in elements) {
+      E e = element as Object/*=E*/;
+      result.add(e);
     }
     return result;
   }
 
   /**
    * Executes a function on each element of the set.
    *
    * The elements are iterated in insertion order.
    */
   void forEach(void action(E element));
 
   /**
    * Provides an iterator that iterates over the elements in insertion order.
    */
   Iterator<E> get iterator;
 }