Relax - Europe will not go Fascist. What will happen though, is that anti-immigration politicians will be elected.
They will be elected, but I doubt they will be able to stop immigration. For one they would have to change the French constitution and the European treaties and directives, which seems impossible (28 agreements needed and more than 10 referendums), or leave the EU altogether. Then they would have to face a reluctant and disobedient administration, protests, legal mug battles, hostile medias, and ridiculous interpretations from constitutional courts.
Besides they would also have to deal with serious diplomatic issues with Muslim countries, who will refuse to take their illegals back. They would likely have to resort to military force to force the hand of those countries, and it could result into a mere strength showcase or escalate into a full-fledged war with the whole Muslim world.
And provided those parties would try to expel a significant number of immigrants, it would also mean a financial effort of many tens or hundreds of billions as those illegals would have to be detained for years until the issues are settled. Add short-term economic damages akin to a couple of years of growth.
I doubt democracy can still change anything significant in an European country. Because of the European mess and because of the rigidity of the European version of human rights and its ridiculous interpretations by constitutional courts. And because of our propaganda (school, medias), censorship (ban on ethnic stats) and judiciary repression (ban on hate speech to silence critics of Islam), all of them which prevent us to get informed and hold serious debates. And because we would have to become a lot more radical than we are, ready to far bigger sacrifices, despite medias promoting moderation, concessions and compromises.
In a recent poll, 49% of French people declared that they would like a strong leader who would not have to care about the elections or the parliament. A dictator in other words.
Personally I only see three ways through which things could change: the introduction of constitutional initiatives (to allow citizens to redact and vote constitutional amendments without parliament interference), a coup from the army to instigate a dictatorship, or armed conflicts.
The latter seems the most likely: I envision a future like Lebanon for France, with Muslims and Frenchmen occasionally slaughtering each other at local scale, with Muslim cities cohabiting alongside French cities, and no government between them because both sides will not be able to agree on anything. Until partition or until one of us disappear through assimilation or extermination.