Move another batch of analyzer tests away from offset-based
assertErrorsInCode() expectations. Put expected diagnostics and context
messages directly in the source snippets instead, including multi-file
cases that need context markers in supporting files.
This keeps each diagnostic expectation next to the code that produces
it, making the tests easier to read and less fragile when surrounding
code changes. It also removes now-unused diagnostic imports and helper
parameters that were only needed for explicit ExpectedDiagnostic lists.
Update the diagnostic expectation utilities so stripping generated
marker lines preserves the retained source lines correctly.
Change-Id: Ibe3890b9ff5e2b2e102e13e5ecc243bdf80c5f4b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/509682
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Konstantin Shcheglov
3a85655f91