Use an ES6 map for a linked identity hash map.

sdk/lib/_internal/compiler/js_lib/isolate_serialization.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of _isolate_helper;
 
 /// Serialize [message].
 _serializeMessage(message) {
   return new _Serializer().serialize(message);
 }
@@ skipping 147 matching lines @@
 
   serializeClosure(Closure x) {
     final name = IsolateNatives._getJSFunctionName(x);
     if (name == null) {
       unsupported(x, "Closures can't be transmitted:");
     }
     return ['function', name];
   }
 
   serializeDartObject(x) {
+    if (!isDartObject(x)) unsupported(x);
     var classExtractor = JS_EMBEDDED_GLOBAL('', CLASS_ID_EXTRACTOR);
     var fieldsExtractor = JS_EMBEDDED_GLOBAL('', CLASS_FIELDS_EXTRACTOR);
     String classId = JS('String', '#(#)', classExtractor, x);
     List fields = JS('JSArray', '#(#)', fieldsExtractor, x);
     return ['dart', classId, serializeArrayInPlace(fields)];
   }
 }
 
 class _Deserializer {
   /// When `true`, encodes sendports specially so that they can be adjusted on
@@ skipping 174 matching lines @@
     var instanceFromClassId = JS_EMBEDDED_GLOBAL('', INSTANCE_FROM_CLASS_ID);
     var initializeObject = JS_EMBEDDED_GLOBAL('', INITIALIZE_EMPTY_INSTANCE);
 
     var emptyInstance = JS('', '#(#)', instanceFromClassId, classId);
     deserializedObjects.add(emptyInstance);
     deserializeArrayInPlace(fields);
     return JS('', '#(#, #, #)',
               initializeObject, classId, emptyInstance, fields);
   }
 }