ext/dom and ext/xmlwriter are not optional dependencies

Author: sebastianbergmann

composer.json

@@ -22,6 +22,8 @@
     },
     "require": {
         "php": "^5.6 || ^7.0",
+        "ext-dom": "*",
+        "ext-xmlwriter": "*",
         "phpunit/php-file-iterator": "^1.3",
         "phpunit/php-token-stream": "^1.4.2 || ^2.0",
         "phpunit/php-text-template": "^1.2",
@@ -34,9 +36,7 @@
         "ext-xdebug": "^2.1.4"
     },
     "suggest": {
-        "ext-dom": "*",
-        "ext-xdebug": "^2.4.0",
-        "ext-xmlwriter": "*"
+        "ext-xdebug": "^2.4.0"
     },
     "autoload": {
         "classmap": [