Make dart:collection strong-mode clean.

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 3ecb82c708e9a252b12eb768d0b118aae1ec9d3c..8e2a2a18c686f8a85672b761c963e66d2be374f2 100644
--- a/sdk/lib/collection/hash_set.dart
+++ b/sdk/lib/collection/hash_set.dart
@@ -121,7 +121,10 @@ abstract class HashSet<E> implements Set<E> {
    */
   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;
   }