minor #28737 [DependencyInjection] Optimize exporting variables (Sander van der Vlugt)

This PR was squashed before being merged into the 4.2-dev branch (closes #28737).

Discussion
----------

[DependencyInjection] Optimize exporting variables

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #28292
| License       | MIT

Store previously resolved variables so that the relatively expensive function [resolveEnvPlaceholders](https://github.com/symfony/symfony/blob/3e7b029524bf724abfd603c17795af39bb289615/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php#L1776) is not called unnecessarily.

The effect on running ```bin/console``` with an empty cache in a big application (200+ env variables) in seconds:

| Before     | After      |
|------------|------------|
| 24.1       | 15.8       |
| 24.5       | 16.0       |
| 24.7       | 16.3       |
| 24.2       | 16.0       |
|  Avg: 24.4 | Avg:16.0   |

Commits
-------

768de2f7d9 [DependencyInjection] Optimize exporting variables

Author: nicolas-grekas

Dumper/PhpDumper.php

@@ -80,6 +80,7 @@ class PhpDumper extends Dumper
     private $addGetService = false;
     private $locatedIds = array();
     private $serviceLocatorTag;
+    private $exportedVariables = array();
 
     /**
      * @var ProxyDumper
@@ -125,6 +126,7 @@ public function dump(array $options = array())
         $this->locatedIds = array();
         $this->targetDirRegex = null;
         $this->inlinedRequires = array();
+        $this->exportedVariables = array();
         $options = array_merge(array(
             'class' => 'ProjectServiceContainer',
             'base_class' => 'Container',
@@ -304,6 +306,7 @@ public function dump(array $options = array())
         $this->inlinedRequires = array();
         $this->circularReferences = array();
         $this->locatedIds = array();
+        $this->exportedVariables = array();
 
         $unusedEnvs = array();
         foreach ($this->container->getEnvCounters() as $env => $use) {
@@ -1768,6 +1771,10 @@ private function export($value)
 
     private function doExport($value, $resolveEnv = false)
     {
+        $shouldCacheValue = $resolveEnv && \is_string($value);
+        if ($shouldCacheValue && isset($this->exportedVariables[$value])) {
+            return $this->exportedVariables[$value];
+        }
         if (\is_string($value) && false !== strpos($value, "\n")) {
             $cleanParts = explode("\n", $value);
             $cleanParts = array_map(function ($part) { return var_export($part, true); }, $cleanParts);
@@ -1789,6 +1796,10 @@ private function doExport($value, $resolveEnv = false)
             }
         }
 
+        if ($shouldCacheValue) {
+            $this->exportedVariables[$value] = $export;
+        }
+
         return $export;
     }
 }