Has Rashi ever heard of sarcasm?

20. So now, let us kill him, and we will cast him into one of the pits, and we will say, 'A wild beast devoured him,' and we will see what will become of his dreams."		כ. וְעַתָּה לְכוּ וְנַהַרְגֵהוּ וְנַשְׁלִכֵהוּ בְּאַחַד הַבֹּרוֹת וְאָמַרְנוּ חַיָּה רָעָה אֲכָלָתְהוּ וְנִרְאֶה מַה יִּהְיוּ חֲלֹמֹתָיו:
and we will see what will become of his dreams: Rabbi Isaac said, This verse says: "Expound on me." [I.e., this verse demands a midrashic interpretation.] The Holy Spirit says thus: They (the brothers) say, "Let us kill him," but the verse concludes: "and we will see what will become of his dreams." Let us see whose word will stand up, yours or Mine. It is impossible that they (the brothers) are saying,"and we will see what will become of his dreams," because, since they will kill him, his dreams will come to nought. [From Tan. Buber, Vayeshev 13]		ונראה מה יהיו חלומותיו: אמר ר' יצחק מקרא זה אומר דרשני, רוח הקודש אומרת כן. הם אומרים נהרגהו, והכתוב מסיים ונראה מה יהיו חלומותיו, נראה דבר מי יקום או שלכם או שלי. ואי אפשר שיאמרו הם ונראה מה יהיו חלומותיו מכיון שיהרגוהו בטלו חלומותיו:

I ask:  Has Rashi (quoting the Medrash of course) never heard of sarcasm?  

Or is he saying that it's wrong to impute sarcastic intent to the brothers' speech patterns? 

Or perhaps he is saying that even though they intended their words to be sarcastic on a literal level, the inclusion of the words in the Torah means that they have greater significance that must transcend bitter humor.  I like this take best, but it stills carries the slight problem that Rashi says "It is impossible that [the brothers] are saying..."