[cfe] Knowledge of whether a `ConstantExpression(<constant>)` is from a deferred library or not

[mkustermann]

One may have code like this:

import 'foo.dart' deferred as foo;
main() {
 ...
 print(foo.topLevelConst);
}

It seems currently this gets lowered into something like this

let
  _ = StaticInvocation(_checkLibraryIsLoaded(...))   // <-- this is modular dart2wasm lowering of `CheckLibraryIsLoaded()` AST node
in
  ConstantExpresion(<constant>)

Now when one visits the AST and hits a ConstantExpression the code generator may want to do something different.

• Normally the code generator has to assume the constant is needed by the main module and has to bundle related data/code in the main module.

• If it knows the constant access is guaranteed to happen after the deferred library was loaded, it can move the constant related data/code into the deferred library.

Right now one needs to match this particular lowering pattern to detect this guarantee, it would be nice if it was more readily available on ConstantExpression.

/cc @johnniwinther

[mkustermann]

And in fact, I encountered situations where the simple pattern recognition fails, e.g.

    let
      final dynamic #t4 = _in::checkLibraryIsLoaded("h0")
    in 
      let
        final 0::H0 #t5 = #C8
      in 
        let 
          final core::int #t6 = 1
        in 
          #t5.{0::H0::c}{(core::int) → void}(#t6){(core::int) → void};

Here the constant use isn't always Let.body == ConstantExpression(...)

So one may need to walk up the parent chain.

[johnniwinther]

cc @chloestefantsova