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..e99f9fb696529d0ae2ccafb70af8311d33c78f62 100644
--- a/pkg/serialization/lib/src/reader_writer.dart
+++ b/pkg/serialization/lib/src/reader_writer.dart
@@ -39,8 +39,8 @@ class Writer implements ReaderOrWriter {
    * once and stored here for each object. This provides some space-saving,
    * but also serves to record which objects we have already seen.
    */
-  final Map<dynamic, Reference> references =
-      new HashMap<Object, Reference>(equals: identical);
+  final Map<dynamic, Reference> references = 
+      new Map<Object, Reference>.identity();
 
   /**
    * The state of objects that need to be serialized is stored here.