You can tell the difference isn’t important if it’s not reflected in the pronunciation of the sentence. Surely your claim isn’t that such a sentence, spoken aloud, would seem ambiguous or incomprehensible to the listener? Besides, it’s clear that it couldn’t be “it’s” since that would constitute some sort of shift between registers that sounds incongruous and unnatural.
At any rate, the application of the term “grammar” to rules of orthography (i.e., writing) is a mistake because it conflates the natural and unavoidable “rules” of language with the artificial rules of writing. Grammar is a term for the rules that native speakers implicitly and unconsciously apply (no matter their education) when they speak, and the grammatical principles underpinning nonstandard dialects (ones in which “ain’t” or double negatives may be used, for example) are just as complicated and just as reflexive as the ones formally taught to students.
Writing, on the other hand, is not governed by a natural capacity of the brain, and its rules are in many ways completely independent of the rules that underlie the language it captures. Knowing how to properly use the apostrophe is important for any educated person, as is a mastery of Standard English in both spoken and written forms, since a person’s language usage is, regrettably or not, a tool used to judge them. (Note: the use of “them” as a gender-neutral third person pronoun is hundreds of years old, and William Shakespeare and Jane Austin are only two of the many great authors who have used the device in their writing.) However, that doesn’t mean that there is any inherent superiority to the “rules” of language as asserted by the fourth-grade teachers of the world. There is ample reason to learn them, but there is no justification for the reverence some people seem to have for them. Most of the rules we’re taught by our teachers don’t describe the language as it’s actually used, even in formal contexts, even by great writers, even hundreds of years ago. The long-standing disgust associated with preposition-stranding, for instance, comes out of a centuries-old misapplication of a grammatical rule of Latin (in which stranded prepositions were indeed impossible) to English. The rules regarding “split infinitives” result from the fact that Latin’s infinitives were single words, and thus simply could not be split. Naturally, the rules of Latin grammar are not valid tools for evaluating the English language - and yet these silly restrictions linger and are still taught to children.
One wonders how the English language managed to survive for hundreds of years without fourth-grade teachers to rap knuckles and click tongues. Shakespeare himself would have earned reprobation for his creative word-formation, inconsistent spellings, and the ending of sentences with prepositions. The average fourth-grade teacher who insists that “proper” English is the mark of civilization has either little familiarity with great literature or such a grandiose self-concept that she considers herself the superior of every great writer in the history of our language.
Fortunately, English (as with all languages) is not the fragile creature lying in a sick-bed that the fourth-grade teachers of the world seem to think it is. Not only did it survive and produce many of its sweetest fruits before their claw-like hands got a hold of it, but it will never be restrained by them; its evolution is not something that can be altered by all the dusty tomes and rulers-turned-weapons the opposing forces can muster.