Fix strong mode error in buffer_utils.sort

[yourpalal]

Fix strong mode error in buffer_utils.sort. Once temp is made, the type information is lost and setRange fails type checks.

You can see this error by running

pub serve --web-compiler dartdevc example

and going to http://localhost:8080/?demo=box_test

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[yourpalal]

I signed it!

[googlebot]

CLAs look good, thanks!

[feroult]

Hi!

No merges so far... Any plans?