Back on March 7 of '08, Norm hosted a session ("East Boston Project") in which he conducted an on-the-job interview of a roofing contractor.
Highlighted, among other things, was this contractor's approach to flashing a dormer.
The contractor used what he called a "flexible chimney corner" (see attached thumbnail).
The "corner" was fastened to the dormer-roof-junction. It looked like, maybe, 8" by 10" (can't be sure) piece of corrugated, flexible aluminum.
I'm in the process of resurfacing my roof and repairing the dormer.
Try as I may, I can't for the life of me find a supplier of these flexible chimney corners.