[build_web_compilers] wasm compilation bug with non-root entrypoint

[schultek]

Description

Having a Dart web project with an entrypoint that is in a subfolder (e.g. '/web/app/main.dart') instead of the root ('/web/main.dart') fails with the new wasm compiler.

The issue is that the main.dart.js always fetches the .wasm file as if it would exist in the root:

Which results in:

• The main.dart.js and main.mjs are correctly fetched from their sub-path.
• The .wasm file is correctly generated at /app/main.wasm, but tried to be fetched from the root.

Steps to reproduce

1. Create new web project with dart create -t web test
2. Update depencency to build_web_compilers: ^4.1.0-beta.0
3. Add build.yaml with

   global_options:
   build_web_compilers:entrypoint:
   options:
     compiler: dart2wasm

4. Move web/main.dart to web/app/main.dart and adjust the script path in index.html accordingly
5. Run dart run build_runner serve and open localhost:8080

Context:

• Dart SDK version: 3.6.0-218.0.dev (dev) (Thu Sep 5 05:03:17 2024 -0700) on "macos_arm64"
• Package: build_web_compilers: ^4.1.0-beta.0

Tagging:

@kevmoo since this could also be a general wasm compilation issue @simolus3 since you worked on this