It looks to me that the first dissolve in fades first the bright parts and then the rest.
I thought that is one of the options in the dissolve transition 'look' dropdown menu.
Don't have any similar material to play with so can't test.
If it's not a dissolve then it might be done via keying out the dark bits, fade in the result via opacity change and then the rest.
But maybe somebody else knows a third party that makes that effect ready to use for you?