Dart deferred loading meta issue

The Dart team is experimenting with using binaryen tools to support our deferred loading feature for users targeting wasm. We currently have experimental support for the feature by having the compiler emit multiple modules.

Typically, when emitting a single module, we use wasm-opt with the --closed-world flag to optimize the wasm module that dart2wasm is emitting. This works well for us and sometimes produces 3x code size improvements as well as other structural improvements like inlining, devirtualization, DCE, etc.

Now with deferred loading we are now splitting that single module into many modules that collectively represent the same program. So we technically still have a closed world... but split across multiple modules. Ideally we could specify all these modules to wasm-opt and it would perform the same (or close to the same) optimizations collectively across all these modules. Today wasm-opt doesn't support anything like this.

I've tried implementing this using a combination of wasm-merge -> wasm-opt -> wasm-split but ran into several issues where the code didn't load and the code size didn't improve much.

Associated issues:
https://github.com/WebAssembly/binaryen/issues/7725
https://github.com/WebAssembly/binaryen/issues/7724
https://github.com/WebAssembly/binaryen/issues/7723
https://github.com/WebAssembly/binaryen/issues/7722
https://github.com/WebAssembly/binaryen/issues/7721

Start by reading the five associated issues, then review how wasm-opt, wasm-merge, and wasm-split are used for the current multi-module deferred-loading workflow. The work is done when Binaryen has a defined and working way to optimize the split modules collectively without breaking loading or losing the expected code-size improvements.