Fixes https://github.com/dart-lang/sdk/issues/61449
The previous code, sure enough, only created a DiagnosticReporter for
the defining unit, and only visited the main unit.
The fix is to create a DiagnosticListener for each unit, set the
DiagnosticReporters correctly and the "current unit" as we go, and then
pull the reported diagnostics from each listener.
Change-Id: I6b645284c58e5e35ccc5b89f30f6ffe97f48296c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/448644
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Sam Rawlins
c667238efd