[js_interop/dart2wasm] Canonicalize and move `<constant-or-literal>.toJS` at compile-time to mjs file

[mkustermann]

We should evaluate whether it makes sense for dart2wasm recognize <constant-or-literal>.toJS expressions and canonicalize & move those objects at compile-time to JS (by emitting them into the ...mjs file and importing those external references) instead of lazily at runtime.

It may be very common to have code such as

@JS()
external JSObject foo(JSString value);

main() {
  foo('foo'.toJS);
}

Currently one may be tempted to cache such manually at the top-level via

@JS()
external JSObject foo(JSString value);

final jsFooArg = 'foo'.toJS;
main() {
  foo(jsFooArg);
}

but this compiles to worse code in dart2js (see #56045)

/cc @srujzs

[rakudrama]

They don't necessarily need to be moved all the way to the mjs file.
'foo'.toJS could be hoisted to a static scope.
Most users are not going to write code like the jsFooArg example, but we could do that automatically.

It would be more powerful to treat this more like 'boxing' and permit CSE, DCE, LICM to effect hoisting

• out of a loop
• out of a closure into its context chain
• to a static scope lazy initializer when the inputs are static scope lazy initialized.

[mkustermann]

This issue isn't that relevant anymore, because we no longer have One/TwoByteStrings and don't have to copy across the boundary. Thay are also no longer in mjs file

The only thing to optimize would be boxes, but in many cases binaryen may already take care of this

[mkustermann]

I'm going to close this as we don't have custom string implementations anymore.

The question around how to avoid boxing of objects is more general, applies also to other types like integers, doubles, etc.