bug symfony#52869 [AssetMapper] Improve link generation script (smnandre)

This PR was squashed before being merged into the 6.4 branch.

Discussion
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[AssetMapper] Improve link generation script

| Q             | A
| ------------- | ---
| Branch?       | 6.4
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Issues        | -
| License       | MIT

Avoid JS console warnings due to missing comma and shorten a bit the code
(document.head is available since IE9)

Felt on it while investigating this HTTPS/2/Push thing with Firefox

(poke `@weaverryan`)

Update: remove the "?" after 7.1 to please carson :)
Update(bis): target 6.4 / bugfix

Commits
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5ad7a1e [AssetMapper] Improve link generation script

Author: nicolas-grekas

src/Symfony/Component/AssetMapper/ImportMap/ImportMapRenderer.php

@@ -78,7 +78,7 @@ public function render(string|array $entryPoint, array $attributes = []): string
                 // importmap entry is a noop
                 $importMap[$importName] = 'data:application/javascript,';
             } else {
-                $importMap[$importName] = 'data:application/javascript,'.rawurlencode(sprintf('const d=document,l=d.createElement("link");l.rel="stylesheet",l.href="%s",(d.head||d.getElementsByTagName("head")[0]).appendChild(l)', $path));
+                $importMap[$importName] = 'data:application/javascript,'.rawurlencode(sprintf('document.head.appendChild(Object.assign(document.createElement("link"),{rel:"stylesheet",href:"%s"}))', addslashes($path)));
             }
         }
 

src/Symfony/Component/AssetMapper/Tests/ImportMap/ImportMapRendererTest.php

@@ -92,7 +92,7 @@ public function testBasicRender()
         $this->assertStringContainsString('"app_css_preload": "data:application/javascript,', $html);
         $this->assertStringContainsString('<link rel="stylesheet" href="/subdirectory/assets/styles/app-preload-d1g35t.css">', $html);
         // non-preloaded CSS file
-        $this->assertStringContainsString('"app_css_no_preload": "data:application/javascript,const%20d%3Ddocument%2Cl%3Dd.createElement%28%22link%22%29%3Bl.rel%3D%22stylesheet%22%2Cl.href%3D%22%2Fsubdirectory%2Fassets%2Fstyles%2Fapp-nopreload-d1g35t.css%22%2C%28d.head%7C%7Cd.getElementsByTagName%28%22head%22%29%5B0%5D%29.appendChild%28l%29', $html);
+        $this->assertStringContainsString('"app_css_no_preload": "data:application/javascript,document.head.appendChild%28Object.assign%28document.createElement%28%22link%22%29%2C%7Brel%3A%22stylesheet%22%2Chref%3A%22%2Fsubdirectory%2Fassets%2Fstyles%2Fapp-nopreload-d1g35t.css%22%7D', $html);
         $this->assertStringNotContainsString('<link rel="stylesheet" href="/subdirectory/assets/styles/app-nopreload-d1g35t.css">', $html);
         // remote js
         $this->assertStringContainsString('"remote_js": "https://cdn.example.com/assets/remote-d1g35t.js"', $html);