PHPUnit 4.8.35 is the lowest version with FC layer.

mondrake · web-flow · commit 9391a41edddd · 2018-01-11T18:42:43.000+01:00
diff --git a/composer.json b/composer.json
@@ -19,6 +19,6 @@
       "bin-dir": "bin"
     },
     "require-dev": {
-        "phpunit/phpunit": "^4.8 || 5.* || 6.*"
+        "phpunit/phpunit": "^4.8.35 || 5.* || 6.*"
     }
 }
