Make dart:collection strong-mode clean.

sdk/lib/collection/linked_hash_set.dart

Index: sdk/lib/collection/linked_hash_set.dart
diff --git a/sdk/lib/collection/linked_hash_set.dart b/sdk/lib/collection/linked_hash_set.dart
index e087b5f630fcc16fbd30cd00a9c8034d8e464feb..8826710ce745ccd9372c1131128f632280c61cd6 100644
--- a/sdk/lib/collection/linked_hash_set.dart
+++ b/sdk/lib/collection/linked_hash_set.dart
@@ -104,8 +104,8 @@ abstract class LinkedHashSet<E> implements HashSet<E> {
    */
   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) {
+      result.add(element as E);
     }
     return result;
   }