Informal spec

Dart 1.0 allows any overridden method to tighten the type of a parameter, which is not statically sound without introducing runtime checks.

Strong mode currently uses @checked. This must be changed to covariant.

Dart2js and the VM should ignore the marker (for now).

For 1.22:

• informal specification: (Merged PR Informal proposal for covariant overrides. #28176) current version
• dart2js Implement covariant marker in dart2js #28165
• vm Implement covariant marker in the VM #28166
• analyzer Support covariant marker in the analyzer #28167
• ddc
  • support covariant syntax. Switch to covariant in DDC #28168
  • change runtime type of tear-offs. This change is backwards-compatible (since it would allow more). covariant parameters should have a wider parameter type at runtime #27484.
• dartfmt Support covariant marker. dart_style#572
• dartdoc Implement covariant marker in dartdoc dartdoc#1322
• changelog updated (pending https://codereview.chromium.org/2648203003/)

At some later point:

• full specification Specify covariant overrides #28164