Make getters and setters actually look like their declarations

[srawlins]

Fixes #1846, almost 6 years old! 😬

Here's a screenshot of what the new signatures look like, with get and set (I hate that the single parameter on the setter gets newlined, but that's a different issue. Maybe track #3721 for that one.

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[srawlins]

CC @dart-lang/analyzer-team

[pq]

Sweet!