const extensions / `constexpr` in Dart

Consider a case in which one may want to instantiate a given `const` instance, in different places, a few too times too many:

```dart
const d = Duration(milliseconds: 250); // <--
withDuration(d); // ...
```

After a (short) while, one may attempt to reach for a short-hand, such as:

```dart
extension DuranDurat on int {
get Duration minutes => Duration(minutes: this);
get Duration secs => Duration(seconds: this);
get Duration ms => Duration(milliseconds: this);
}
```

This one, however - would no long evaluate at compile time.

Having a:

```dart
extension ConstDurat on int {
const get ms => Duration(milliseconds: this);
}
```

Which would de-sugar into the usual:

```dart
const Duration(milliseconds: ...)
```

at every invocation inline, would be rather useful indeed. The C++/like `constexpr` were brought up already at #2222, yet I don't see much discussion around it. Low priority as it might be, it wouldn't hurt the language in the slightest.

Start by reading the proposal and the referenced issue #2222; the payload names no source files, tests, or entry points. Done would require a language-design decision on const extension getters or constexpr-like syntax, followed by an agreed specification and implementation plan.

Written by the indexing model from the issue text.