The problem with this christian assertion is that it isn't read in context. If one truly takes the time to study the whole chapter in context they will see that Yeshua is not making the declaration that he is God and they will also find that the reason they wanted to kill him was because he said he existed before Abraham. Back in verse 8:37 is where we first hear Yeshua saying to them that they wanted to kill him. So it wasn't in 8:58. They wanted to kill him way before he said he existed before Abraham. The focus of the conversation is on Abraham. The descendants of Abraham had a great respect and love for him so for some young man to come along appearing to boast that he was greater than Abraham or knew Abraham before he existed was a great disrespect to them. An understanding of the greek that is being used clarifies that.
John 8:58
Jesus answered, "The truth is, I existed before Abraham was even born!
Strongs Greek Lexicon
ego
1473. ego eg-o' a primary pronoun of the first person I (only expressed when emphatic):--I, me.
eimi
1510. eimi i-mee' the first person singular present indicative; a prolonged form of a primary and defective verb; I exist (used only when emphatic):--am, have been, X it is I, was
For me the question is, did Yeshua actually exist before Abraham and if he did does he reveal to us that he was God? Well to answer that question I look to this verse;
John 17:5
And now, O Father, glorify you, me with your own self with the glory which I had with you before the world existed.
So the answer is yes he existed before Abraham but No, he does not reveal he is God rather he was with his god before the world was created. Additionally if you look at 17:5 at the end is that word (existed). Some render it as (was). It is the same word that is in John 8:58 (am). They mean the same thing. Yeshua (existed before Abraham). There was no declaration made by him calling himself God.
In Islam God is God and that's it. All of the prophets are considered rasullulah (Isa rasulullah, Muhammad rasulluah, Musa rasullulahh, Ibrahim rasullulah). Are they considered God or God in the flesh? No. Islam teaches that these men were blessed by Allah to speak the word of Allah.
Now one could make the same Argument that other prophets in the OT were God in the flesh and this theory was not exclusive to Yeshua. Reading the 1st chapter in the book of Haggai gives one the impression that Haggai is God incarnate. Do Jews or Christians believe that he is. Nope. In that book he is called God's Messenger and God's Prophet just like in Islam. Yeshua says the same thing about himself. He calls himself the servant of the master, the one that was sent. He calls himself a prophet as well.
John 3:34
For he whom God has sent speaks the words of God: for God gives not the Spirit by measure unto him.
John 13:16
Verily, verily, I say unto you, The servant is not greater than his master; neither he that is sent greater than he that sent him.
Luke 13:13
Nevertheless I must walk to day, and to morrow, and the day following: for it cannot be that a prophet perish out of Jerusalem.
If we look upon Yeshua with mystery then we will be mystified but if we look upon him with open eyes then the simple truth as to who he was and who sent him is revealed clearly with no mystery.