In a formal sense, I doubt there is a direct antonym. I would love to be proven wrong, as there are many times when such a word would be useful, as we refer to the lack of context frequently and dictionaries often consider the meanings of words in isolation, yet I have never heard one word that can refer to the concept. The Roget's 21st Century Thesaurus, Third Edition available at thesaurus.com, which as a matter of policy, you probably should've checked prior to asking us, doesn't have one for instance.
The morphologically congruent antonym would be distext, because the con- part of context is a latin wordforming element meaning with or together, whereas dis- is a latin wordforming element meaning separate or apart according to Webster's Revised Unabridged Dictionary 1913. If you think English should be regarded as a productive language, that seems to be most likely candidate. However there are other considerations.
No such word as distext appears in that edition of Webster's, the entire Oxford English Dictionary., onelook or Google nGrams. Onelook is a dictionary search engine that searches all of the other major dictionaries available online and hence the most reputable of printed dictionaries available at this point in time as well, such as Random House Kernerman Webster's College Dictionary, The American Heritage Dictionary of the English Language 5th edition, Merriam-Webster, Wiktionary and many more.
As far as cognative transparancy goes, I'm not so sure it would be easily understood either. Most English speakers today percieve the dis- wordforming element as a simple negation, as indicated by the first listed definition in The American Heritage Dictionary Fifth Edition's definition of dis- as meaning "not" which is somewhat different. Whereas context is how a word is considered together with the text, distext might be perceived as trying to say that the word is not text, rather than saying that the word is being considered in isolation or apart from the text.
With no evidence of prior use, it would seem to be be a nonce word or a neologism at best. It doesn't quite serve the purpose of a word if it isn't immediately recognizable too. If the most logical candidate for such a word does not exist in the language, it is doubtful any less likely alternative exists as well.