Update dart:collection.

tool/input_sdk/lib/collection/hash_set.dart

Index: tool/input_sdk/lib/collection/hash_set.dart
diff --git a/tool/input_sdk/lib/collection/hash_set.dart b/tool/input_sdk/lib/collection/hash_set.dart
index 668ad0fb12860bc50500f0e2bdae702ffcb8c6d2..8e2a2a18c686f8a85672b761c963e66d2be374f2 100644
--- a/tool/input_sdk/lib/collection/hash_set.dart
+++ b/tool/input_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 (final e in elements) result.add(e as E);
+    for (final e in elements) {
+      E element = e as Object/*=E*/;
+      result.add(element);
+    }
     return result;
   }