"Shouting 'Fire!' in a crowded theater" is one of the laziest tropes in First Amendment law and commentary, as @AlanDersh argued in this classic 1989 Atlantic essay. theatlantic.com/past/docs/issu…
The analogy, Dersh rightly notes, was inapt when Justice Holmes came up with it in Schenck v. U.S. (1919), and it is inapt almost every time it has been cited since.
To his list of close analogies, one might add sending a fake bomb or harmless white powder through the mail. The purpose is to provoke panic, even if there's no actual physical danger. 
Dersh also lists some "more distant" analogies: incitement, fighting words and disruptive heckling.
Now, here's a piece that accuses Trump of "shouting 'fire.'" The author twice characterizes Trump's speech as incitement. nydailynews.com/opinion/ny-ope…
Incitement, however, has a specific legal definition, under Brandenburg v. Ohio (1969). It must be "directed to inciting or producing imminent lawless action" AND "likely to incite or produce such action."
Arguably Candidate Trump engaged in incitement when he seemed to encourage supporters to rough up hecklers at rallies.
A prosecutor might find it difficult to prove both elements, however, unless someone actually followed his urging. If no one did, he could say he meant it jokingly and his audience understood it that way.
Maxine Waters's urging supporters to harass cabinet members also would not qualify as incitement, even if someone did act on it, since imminence is missing from both elements.
And contributing to a "poisonous atmosphere"--which Trump has undoubtetly done, but so have a great many other politicians, political actors and commentators--comes nowhere near meeting the definition of incitement.
End of rant.
