Proposal: Allow "late final" on const classes and enhanced enums

Today, it is unintuitive how to support lazy/memoized fields on const classes or enhanced enums:

```dart
enum Ansi8BitColors {
black,
red,
green,
yellow,
blue,
magenta,
cyan,
white;

// This is a simple example, imagine something more runtime/memory intensive.
late final escapeCode = '\u001b[${index + 30}';
}
```

The most obvious pattern to me is using an expando:

```diff
enum Ansi8BitColors {
black,
red,
green,
yellow,
blue,
magenta,
cyan,
white;

+ static final _escapeCodes = Expando();
- late final escapeCode = '\u001b[${index + 30}';
+ String get escapeCode => _escapeCodes[this] ??= '\u001b[${index + 30}';
}
```

However, that just seems unnecessary. Could const classes/enhanced enums just support this out of the box?

Start by reading the proposal and its enum example, then review the surrounding Dart language rules for const classes, enhanced enums, and late final fields. Done means the language-design question has a decided specification outcome and the documented behavior covers the proposed lazy or memoized fields.

Written by the indexing model from the issue text.