Use an ES6 map for a linked identity hash map.

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

 // Copyright (c) 2012, 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.
 
 library _isolate_helper;
 
 import 'dart:_js_embedded_names' show
     CLASS_ID_EXTRACTOR,
     CLASS_FIELDS_EXTRACTOR,
     CREATE_NEW_ISOLATE,
     CURRENT_SCRIPT,
     GLOBAL_FUNCTIONS,
     INITIALIZE_EMPTY_INSTANCE,
     INSTANCE_FROM_CLASS_ID;
 
 import 'dart:async';
 import 'dart:collection' show Queue;
 import 'dart:isolate';
 import 'dart:_native_typed_data' show NativeByteBuffer, NativeTypedData;
 
 import 'dart:_js_helper' show
     Closure,
     InternalMap,
     Null,
     Primitives,
     convertDartClosureToJS,
+    isDartObject,
     random64,
     requiresPreamble;
 
 import 'dart:_foreign_helper' show DART_CLOSURE_TO_JS,
                                    JS,
                                    JS_CURRENT_ISOLATE_CONTEXT,
                                    JS_CURRENT_ISOLATE,
                                    JS_EMBEDDED_GLOBAL,
                                    JS_SET_CURRENT_ISOLATE,
                                    IsolateContext;
@@ skipping 1405 matching lines @@
   }
 
   bool operator==(Object other) {
     if (identical(other, this)) return true;
     if (other is CapabilityImpl) {
       return identical(_id, other._id);
     }
     return false;
   }
 }