This fixes how the sample analyzer handles missing_identifier errors. It was looking at the wrong line, and missing an else clause, so it was silently allowing missing_identifier errors to pass.
In addition, this fixes the sample generation so that it uses the correct filename for the output files: it previously was looking for the first line that had a filename, which was meant to indicate a non-generated line. This change adds a new Line.generated constructor for generated lines, so that they can also have the correct filename associated with them.
This switches the sample analysis code to use package:flutter_lints instead of the flutter repo analysis options, so that they are compatible with a similar change to DartPad.
This turns off the "directives_ordering" lint when analyzing samples, since it's indeterminate what the ordering will be, given that samples derive their imports both from the template and the example code.
This is temporary to avoid broken builds, but the correct solution is to reorder the output so that the imports are ordered properly so that we give the proper example for our users.
