What premonition does Romeo have in "Romeo and Juliet"?

In Act 5, scene 1, Romeo declares the following:

"If I may trust the flattering truth of sleep,
My dreams presage some joyful news at hand:
My bosom's lord sits lightly in his throne;
And all this day an unaccustom'd spirit
Lifts me above the ground with cheerful thoughts.
I dreamt my lady came and found me dead--
Strange dream, that gives a dead man leave
to think!--
And breathed such life with kisses in my lips,
That I revived, and was an emperor.
Ah me! how sweet is love itself possess'd,
When but love's shadows are so rich in joy!"

This is clearly a premonition that he has had. The irony is that the reality does not have the joyful ending predicted in the dream. The dream has made Romeo light in spirit - it has pleased him and put him in a less morose mood. He is now in a different frame of mind. The dream is so unusual that it would drive even a dead man to think! However, even though Juliet does in reality find him dead later when she awakens from her death-like slumber, her kisses do not revive him and she, instead, commits suicide.

Tragically, it is immediately after this declaration that Romeo is informed by Balthasar about Juliet's 'death' and her burial. He then plots his own demise by purchasing an extremely powerful poison from an apothecary. 

Earlier in the play, In Act 1, scene 4, Romeo does express some misgivings about attending the Capulet ball and, although he accurately predicts the events to follow, this is not, in the true sense of the word, a premonition. Romeo is merely expressing a feeling or thought as one would when one is about to take a risk, as they were at the time, by venturing to an occasion hosted by their enemies.