Sort named fields in mini types record representation.

This brings the "mini types" representation of records (which is used
for _fe_analyzer_shared unit tests) into alignment with the behavior
of analyzer and CFE record types (which sort their fields), and makes
the representation more consistent with that of function types.

Since all implementations of record types now maintain named fields in
sorted order, documentation has been added to
SharedRecordTypeStructure to indicate that the fields returned by the
`namedTypes` getter are sorted.

This exposed a minor bug in the error recovery logic in the shared
pattern type analyzer: upon encountering a record pattern with
duplicate field names, after reporting the appropriate error, it would
nonetheless attempt to create a record type containing duplicate
fields (potentially breaking the assumptions made by other code that
handles record types). This bug was fixed by adjusting the record
pattern analysis logic so that it drops duplicate field names when
constructing record types.

Change-Id: Ib06b86df980afcf17896e10ac7856f994aeda86f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/388041
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>