Shakespeare's Romeo and Juliet doesn't begin with foreshadowing, but with foretelling.
CHORUS. ...From forth the fatal loins of these two foes
A pair of star-cross'd lovers take their life;
Whose misadventur'd piteous overthrows
Doth, with their death, bury their parents’ strife. (Prologue, 5-8)
The Chorus tells the audience what's ultimately going to happen. There's no mystery about that. What the audience doesn't know is how it's going to happen, or if anything else unrelated to the "star-cross'd" lovers' deaths might happen along the way.
However, Shakespeare does manages to infuse a little bit of foreshadowing into the Chorus's foretelling of the end of the play.
CHORUS. A pair of star-cross'd lovers take their life... (Prologue 5)
This is a very subtle play on words, the meaning of which becomes clear only when Romeo and Juliet take their own lives—if anybody actually remembers what the Chorus said in the fifth line of the play while they're watching Romeo and Juliet kill themselves "two hours" later.
With this Prologue, Shakespeare sets up quite a challenge for himself. Since he tells the audience what happens at the end of the story, Shakespeare has to write a really good play to keep the audience's interest, particularly since the basic story of Romeo and Juliet was well-known to his audience.
There had been three fairly recent publications of the story of Romeo and Juliet, including Arthur Brooke’s narrative poem, The Tragicall Historye of Romeus and Juliet, published in 1562—which scholars consider the primary source of Shakespeare's play—as well as Giuletta e Romeo, by Matteo Bandello, published in 1554, and Luigi da Porto’s Giulietta e Romeo, published in 1530. Other theatre companies in London were also performing their own versions of the Romeo and Juliet story.
Nevertheless, the question arises that if the audience knows what's ultimately going to happen, is the foreshadowing in the play really foreshadowing, or is it simply a reminder of what the audience has already been told?
Most of the faux-foreshadowing in the play involves the deaths of Romeo and Juliet, but even what seems like foreshadowing of events affecting other characters really isn't.
In act 3, scene 1, Benvolio cautions Mercutio that that they should get out of the streets to avoid meeting any Capulets:
BENVOLIO. I pray thee, good Mercutio, let's retire.
The day is hot, the Capulets abroad.
And if we meet, we shall not scape a brawl... (3.1.1-3)
There is a brawl, of course—which comes as no real surprise to anybody—in which Mercutio is killed by Tybalt, and Tybalt is killed by Romeo,
Tybalt seems to have foreshadowed this unfortunate turn of events at the Capulet's feast in act 1, scene 5, when Lord Capulet tells Tybalt simply to endure Romeo's presence, and not cause any trouble.
TYBALT. Patience perforce with wilful choler meeting
Makes my flesh tremble in their different greeting.
I will withdraw; but this intrusion shall,
Now seeming sweet, convert to bitt'rest gall. (1.5.94-97)
The confrontation between and among Tybalt, Mercutio, and Romeo was inevitable, and Tybalt simply foretold that something bad was going to happen, which it did. The audience might not have foreseen the extent to which something bad was going to happen, but they certainly knew that something bad would happen.
The situation is much the same as in Hamlet, after Hamlet tells Horatio what the ghost of his father said to him.
HAMLET. There's ne'er a villain dwelling in all Denmark
But he's an arrant knave.
HORATIO. There needs no ghost, my lord, come from the grave
To tell us this. (1.5.134-138)
Foreshadowing isn't foreshadowing if the audience knows what's going to happen, or if what happens—like a serious confrontation in the street between sword-carrying major characters from warring families—can easily be foreseen.