Record named field shorthand

This is a simple feature. Since we have pattern matching and destructuring of records, I propose that we have a shorthand for adding named fields to record literals in the case of declared identifiers. For example:

```dart
int x = 3;
int get y => 4;

print((:x, :y)); // (x: 3, y: 4)
```

No files, tests, or implementation entry points are named. Start by reviewing the proposed syntax alongside Dart's existing record pattern-matching and destructuring behavior; done means the shorthand's syntax and semantics are fully designed and the required implementation work is identified.

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