Get rid of several old collection workarounds in serialization

pkg/serialization/lib/src/reader_writer.dart

Index: pkg/serialization/lib/src/reader_writer.dart
diff --git a/pkg/serialization/lib/src/reader_writer.dart b/pkg/serialization/lib/src/reader_writer.dart
index 920318555b2e00774d5aed8e74b81fb517f707e7..3ee95a95ff147ad4549e6b8343e3e8b11a95e0f2 100644
--- a/pkg/serialization/lib/src/reader_writer.dart
+++ b/pkg/serialization/lib/src/reader_writer.dart
@@ -40,7 +40,7 @@ class Writer implements ReaderOrWriter {
    * but also serves to record which objects we have already seen.
    */
   final Map<dynamic, Reference> references =
-      new HashMap<Object, Reference>(equals: identical);
+      new HashMap<Object, Reference>.identity();
 
   /**
    * The state of objects that need to be serialized is stored here.