[dart2wasm] Convert simple async function bodies to sync `Future.value` calls

This transforms functions like

    async foo() => const ...;

to

    foo() => Future.value(const ...);

The transformation is done when the async function body is a `const`
expression or a basic literal (string, int, double, bool).

These expressions don't have side effects and they cannot throw, so it's
safe to convert them to `Future.value`s.

This makes the generated code in the ACX demo 2.5% smaller: (`-O4` with
symbol names removed)

- Before: 9,040,020 bytes
- After: 8,805,471 bytes
- Diff: -234,549 bytes, -2.59%

With this we also remove the same special case in the backend to avoid
generating a state machine for these functions, as the special case
handled before the backend now and backend never sees this kind of
functions.

(Technically with inlining or other backend optimizations it could still
see these cases, but the pattern it matches is too strict, and currently
the special case in the backend doesn't do anything on the ACX demo.)

Note: I tried implementing the same in dart2js's await lowering pass in
https://dart-review.googlesource.com/c/sdk/+/422061 and reusing that
pass in https://dart-review.googlesource.com/c/sdk/+/420140.

However while that transformed simple programs as expected, in ACX demo
it still introduced a lot of `Future.sync` calls and made the overall
binary larger. I think we never want to introduce `Future.sync` calls
(at least until we improve code size for closures, see relevant issue
https://github.com/dart-lang/sdk/issues/60458), so for now we don't
reuse dart2js's pass.

Issue: https://github.com/dart-lang/sdk/issues/60433
Change-Id: I206ac8c6081201041f67e7fc91776077e52180a0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/422120
Reviewed-by: Nate Biggs <natebiggs@google.com>
Commit-Queue: Ömer Ağacan <omersa@google.com>