Skip one of tests in ContainerBuilderHasAliasConstraintTest for Symfony <3.3

Author: stloyd

Tests/PhpUnit/ContainerBuilderHasAliasConstraintTest.php

@@ -4,6 +4,7 @@
 
 use Matthias\SymfonyDependencyInjectionTest\PhpUnit\ContainerBuilderHasAliasConstraint;
 use PHPUnit\Framework\TestCase;
+use Symfony\Component\DependencyInjection\ChildDefinition;
 use Symfony\Component\DependencyInjection\ContainerBuilder;
 
 class ContainerBuilderHasAliasConstraintTest extends TestCase
@@ -84,6 +85,10 @@ public function it_expects_a_string_for_service_id()
      */
     public function it_does_not_change_case_of_aliased_service_ids()
     {
+        if (!class_exists(ChildDefinition::class)) {
+            $this->markTestSkipped('This test requires at least Symfony 3.3');
+        }
+
         $containerBuilder = new ContainerBuilder();
         $containerBuilder->setAlias('Interface', 'InterfaceImplementationService');
         $constraint = new ContainerBuilderHasAliasConstraint('Interface', 'InterfaceImplementationService');