minor symfony#25302 [DependencyInjection] Add tests to symfony#25264 (Jean85)

This PR was merged into the 3.4 branch.

Discussion
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[DependencyInjection] Add tests to symfony#25264

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | adds tests for symfony#25264
| License       | MIT

As asked by @nicolas-grekas in symfony#25253 (comment), I've added a test about his PR symfony#25264.

Commits
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93441c1 Add test case for symfony#25264

Author: nicolas-grekas

src/Symfony/Component/DependencyInjection/Tests/ContainerTest.php

@@ -538,6 +538,16 @@ public function testReplacingAPreDefinedServiceIsDeprecated()
 
         $this->assertSame($bar, $c->get('bar'), '->set() replaces a pre-defined service');
     }
+
+    /**
+     * @group legacy
+     * @expectedDeprecation The "synthetic" service is private, replacing it is deprecated since Symfony 3.2 and will fail in 4.0.
+     */
+    public function testSetWithPrivateSyntheticServiceThrowsDeprecation()
+    {
+        $c = new ProjectServiceContainer();
+        $c->set('synthetic', new \stdClass());
+    }
 }
 
 class ProjectServiceContainer extends Container
@@ -565,8 +575,12 @@ public function __construct()
         $this->__foo_bar = new \stdClass();
         $this->__foo_baz = new \stdClass();
         $this->__internal = new \stdClass();
-        $this->privates = array('internal' => true);
+        $this->privates = array(
+            'internal' => true,
+            'synthetic' => true,
+        );
         $this->aliases = array('alias' => 'bar');
+        $this->syntheticIds['synthetic'] = true;
     }
 
     protected function getInternalService()