An interesting question here would be how many of those hundreds of failures were actual bugs and how many were just were just side effect of changing "dynamic is both top and bottom" rule.
So far I have not encountered a single actual issue when migrating users' code to Dart 2 - I encountered "unsoundly" typed code, but it actually worked and did not contain bugs.
1. Is developed at a company with a stronger-than-average culture around quality and tests.
2. Already has copious tests.
3. Is mostly in applications that have been shipped in production.
So most of the bugs were already flushed out before we got to the point where we started tightening things. The more interesting question, which we don't have data for, is how much time it took to flush those bugs out the first time and how much the stricter runtime checking affects that. Hopefully it helps users find these kinds of issues faster.