a === b -> identical(a, b)

runtime/lib/array_patch.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.
 
 // Note that the optimizing compiler depends on the algorithm which
 // returns a _GrowableObjectArray if length is null, otherwise returns
 // fixed size array.
 patch class _ListImpl<E> {
   /* patch */ factory List([int length = null]) {
-    if (length === null) {
+    if (length == null) {
       return new _GrowableObjectArray<E>();
     } else {
       return new _ObjectArray<E>(length);
     }
   }
 
   /* patch */ factory List.from(Iterable<E> other) {
     _GrowableObjectArray<E> list = new _GrowableObjectArray<E>();
     for (final e in other) {
       list.add(e);
     }
     return list;
   }
 }