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

sdk/lib/_internal/compiler/implementation/types/concrete_types_inferrer.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.
 
 part of types;
 
 class CancelTypeInferenceException {
   final Node node;
   final String reason;
 
@@ skipping 1118 matching lines @@
     }
     return type;
   }
 
   ConcreteType visitOperator(Operator node) {
     inferrer.fail(node, 'not yet implemented');
   }
 
   ConcreteType visitReturn(Return node) {
     final expression = node.expression;
-    return (expression === null)
+    return (expression == null)
         ? new ConcreteType.singleton(const NullBaseType())
         : analyze(expression);
   }
 
   ConcreteType visitThrow(Throw node) {
     if (node.expression != null) analyze(node.expression);
     return new ConcreteType.empty();
   }
 
   ConcreteType visitTypeAnnotation(TypeAnnotation node) {
@@ skipping 299 matching lines @@
   }
 
   void internalError(String reason, {Node node}) {
     inferrer.fail(node, reason);
   }
 
   ConcreteType visitTypeReferenceSend(Send) {
     return new ConcreteType.singleton(inferrer.baseTypes.typeBaseType);
   }
 }