"There'll be magic" -- Foreshadowing in For the First Time in Forever?

There doesn't appear to be any foreboding in the scene.

Multiple interviews show that the purpose of the song was to both display a quirky, fun Disney princess and showcase Kristen Bell's "modern. funny girl sensibility" (to quote from the second article).

Consider this segment of an interview with Robert Lopez and Kristen Anderson-Lopez, the songwriters, from the first article linked:

Interviewer So, you know, it's a fun song. There's a line in it that is definitely I think not for children but for adults...
And the line is, don't know if I'm elated or gassy. But I'm somewhere in that zone.

Anderson-Lopez: ...Don't know if I'm elated or gassy. But I'm somewhere in that zone. Again, we were trying to say this is a Disney Princess who gets gas. This is a Disney princess who is human with a human body and we're going to spend some time with a real girl.

In other words, the emphasis was heavily on taking the usual stereotypical Disney princess and making her seem fun, quirky, girl-next-door-esque.

The line about magic was simply a young woman expressing her joy at the gates to the castle finally being opened after all these years.

It does of course accurately forebode what follows, but whether this was intentional (i.e. chosen by Disney), slipped in (i.e. by the songwriters) or just a complete coincidence, is something we'll likely never have an answer for.

However, given the context of the song, it seems very likely it's just a random lyric and isn't tied to any sense of revelation over Elsa's actions.