Add test case for strict types declaration

tests/Composer/Test/Plugin/Fixtures/plugin-v9/Installer/Plugin.php

@@ -0,0 +1,18 @@
+<?php
+
+declare(strict_types = 1);
+
+namespace Installer;
+
+use Composer\Composer;
+use Composer\IO\IOInterface;
+use Composer\Plugin\PluginInterface;
+
+class Plugin implements PluginInterface
+{
+    public $version = 'installer-v9';
+
+    public function activate(Composer $composer, IOInterface $io)
+    {
+    }
+}

tests/Composer/Test/Plugin/Fixtures/plugin-v9/composer.json

@@ -0,0 +1,12 @@
+{
+    "name": "plugin-v9",
+    "version": "9.0.0",
+    "type": "composer-plugin",
+    "autoload": { "psr-0": { "Installer": "" } },
+    "extra": {
+        "class": "Installer\\Plugin"
+    },
+    "require": {
+        "composer-plugin-api": "^1.0"
+    }
+}