How are lines 8 and 12 in Sonnet 18 by Shakespeare a bit of foreshadowing?

In "Sonnet 18" the speaker is comparing a person to a
summer's day and stating that this person is MORE lovely and MORE temperate.  That by
itself is rather surprising because we generally think of summer as the most lovely of
seasons, but the speaker goes on to explain.  He reminds us that summer sometimes has
rough winds, cloudy skies, and is too hot.  He is building to his ultimate point in
lines 7 and 8.  Here he says

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 And every fair from fair sometimes
declines,

By chance or nature's changing course
untrimmed. 

What he is
foreshadowing is that all things, especially beautiful things eventually fade away. 
People age and seasons change.  This point sets up the rest of the sonnet.  Lines 9-12
speak specifically about the person the speaker is addressing.  He says the beauty of
this person will never fade and that death will not be able to brag about him or her
because this poem ("eternal lines to time") will live on and in it, the beauty of the
person will live on so long as the poem exists.  As with most sonnets, the ultimate
point of the poem comes in the last lines.  In this case, it would seem the the speaker
is right.  Here we are more than 400 years after this sonnet was written thinking about
the beauty of the person being discussed.  We are doing that because the "lines" have
lasted that long.