If you have GenArts Sapphire, S_WipeDots does exactly this out of the box.
If you have Trapcode Form, you could use it to control a grid of circles in a precomp, then use that as a luma matte for your transition.
If you have neither of these third-party effects, you could animate a single white circle going from zero to full size over a black background in a precomp. Create another composition containing a grid of these precomps, then use time-remapping or the Time Displacement effect to stagger them in time. This can be automated a bit if you're familiar with expressions.
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