Non-growable list literal syntax

Non-growable lists are implemented more efficiently than growable lists, but creating one is rather verbose and inconvenient.

```dart
// Growable:
var items = foo.map(toItem).toList();
// Non-growable:
var items = foo.map(toItem).toList(growable: false);

// Growable:
var items = [
...firstThings.map((e) => e.foo),
...secondThings.map((e) => e.foo),
if (condition) lastThing,
];

// Non-growable:
var intermediate1 = firstThings.map((e) => e.foo);
var intermediate2 = secondThings.map((e) => e.foo);
// Oops, we cannot use List() any more.
var items = List(intermediate1.length + intermediate2.length + 1);

// Concatenate the Iterables?
var items = List.of(
intermediate1.followedBy(intermediate2).followedBy([lastThing]),
growable: false);
```

Oh! to give up our precious and novel UI-as-code, our spreads and for-elements and if-elements... ☹️ And we're forced to create an intermediate 1-element list with `followedBy([lastThing])`, and I didn't even implement the `if (condition)` part.

### Solution

What a beautiful, space-efficient, fun and quirky world we'd live in, if we had a non-growable list literal syntax. I'm thinking:

```dart

// growable: true
var items = foo.map(toItem).toList();
// growable: false
var items = [...foo.map(toItem)]gf;

// Growable:
var items = [
...firstThings.map((e) => e.foo),
...secondThings.map((e) => e.foo),
if (condition) lastThing,
];

// Non-growable:
var items = [
...firstThings.map((e) => e.foo),
...secondThings.map((e) => e.foo),
if (condition) lastThing,
]gf;
```

Or if we want to reserve `gf` for a gluten-free option, we can use `[]ng` ("non-growable"), `[]f` ("fixed-length"), whatever. A preceding `f` would be great (`f[]`), but I can't see how that wouldn't blatantly conflict with index-expressions.

Start with issue #2477 and read the full comment discussion, focusing on the proposed non-growable list literal forms and their interaction with spreads, for-elements, and if-elements. Done would require an agreed syntax and corresponding Dart language-design decision; no implementation files or tests are identified in the payload.

Written by the indexing model from the issue text.