Issue for tracking that the feature spec is in progress or is complete.

The working spec is located here: https://github.com/dart-lang/language/blob/main/working/wildcards/feature-specification.md

@eernstg @munificent Can we confirm that the spec is basically complete? Is there anything else we need to discuss or decide on?

Work Items

• [Wildcard Variables][spec] Allow super._? language#3792
• [Wildcard Variables][spec] Treat all multiple underscores as wildcards? language#3793