An ending interpretation that I think is worth considering...

Ilya overdosed and died in the McDonald's bathroom. Harley's reborn romance with Ilya was an emotional dream, if you will. Included within it was Ilya's immolation, which was a symbolic death - an attempted reminder from her subconscious that "he is dead, Harley." Her waking up and desperately searching for an Ilya who is no longer there is her finally coming to physical terms with his passing.

When we next see her walk into the Dunkin Donuts, time has actually gone by since Ilya's OD death in McDonalds, her fantasy romance and running away on the bus, and the previous shot of her returning home on the highway. The final scene is intended to show us that after some time, she has finally moved on, and those seated at the table with her are her normal. Her companions' casualness and lack of acknowledgement to her arrival in Dunkin Donuts (despite what would have otherwise been interpreted as very recent events) can attest to this. Despite his impaired state and the group's social habits, I just can't accept that Mike would have gone from being so desperate to reach her to not even noticing or seeming to care that she is now present - especially when she never responded to his calls.

A few things to consider:
-When she returns to Mike and Antoine's, Antoine asks, "Is Ilya all right?", to which Harley quickly responds, "Yeah he's all right." When Mike tries to reach her later, he's under the impression that Ilya is in fact still alive and she has run away with him - because that's all Antoine was privy to.
-Ilya's throwing the cell phone into the air and it turning into a firework was an element of fantasy - a detail to indicate that the transpiring events are not reality.
-No one, including the bus driver, genuinely seemed to have any idea who she was talking about when she was searching for Ilya. Having someone get up mid-trip and demand that the bus be pulled over on the highway would have hardly been a forgettable event for any of the passengers, let alone the driver. I find it hard to believe that while she was sleeping, they all would have then conspired and agreed to pretend they knew nothing when she woke up. Also, with Ilya, the bus driver seemed to concede remarkably easily to pulling over...
"Yo I wanna get off."
"Sir the bus is moving. Return to your seat."
"I want to get off here now."
"You got it!"
...in contrast to how he responded to Harley. While one who sees all of these events as reality could interpret this as buckling to the more dangerous/volatile-looking individual, another who sees only Harley's interaction as reality can interpret this as an illustration of the separation between fantasy (Ilya's interaction) and reality (Harley's interaction).
-As for her other interactions (the drug deal, shoplifting, etc.), whether they ever happened in reality or even how they happened is all conjecture, if one is accepting that it's fantasy.

Harley needed this. She needed it all to finally let go, break free, and become her own person. Upon accepting this interpretation, one could argue that the entire growth of her character - the growth, point, and progress that I've noticed several media critics have posited are absent - occurred in these final scenes of the film.