fix(compiler): preserve bundler directive comments in es5 output

[Arul1998]

What is the current behavior?

GitHub Issue Number: #4122

The Stencil runtime's lazy loader (src/client/client-load-module.ts) annotates its dynamic import() with bundler directive comments — /* @vite-ignore */, /* webpackInclude: ... */, /* webpackExclude: ... */, and /* webpackMode: "lazy" */ — so that downstream bundlers handle the non-statically-analyzable import correctly.

With buildEs5 enabled, those comments are lost in the dist/esm-es5 output. They survive the TypeScript ES5 transpilation, but are then stripped by terser: getTerserOptions doesn't configure format.comments, so terser's default applies, which only keeps license-style comments (@license, @preserve, /*! ... */).

As a result:

• Vite emits a warning for every consumer of an es5-enabled library: The above dynamic import cannot be analyzed by Vite. (this is the warning reported in bug: buildEs5 emits warning with Vite #4122)
• webpack silently loses its webpackInclude/webpackExclude/webpackMode lazy-loading hints in the es5 output.

What is the new behavior?

The ES5 minification options now pass terser a format.comments pattern that preserves bundler directive comments (@vite-ignore and webpack*: magic comments) in addition to terser's default license-comment behavior. The directives now ship in the esm-es5 output, Vite no longer warns, and webpack's hints are honored.

This is a compiler-only change: it does not add any code to the runtime, and modern (non-es5) output is unaffected. Regular comments are still stripped from es5 output as before.

Documentation

N/A

Does this introduce a breaking change?

• Yes
• No

Testing

• Added unit tests in src/compiler/optimize/test/optimize-module.spec.ts covering the ES5 path: bundler directive comments are preserved inside the dynamic import(), ordinary line/block comments are still removed, and license comments are still preserved.
• Manually verified end-to-end: built a component library with buildEs5 enabled against this branch and confirmed the directive comments are present in dist/esm-es5. Ran the output through Vite's dev-server transform (vite:import-analysis): the current published output reproduces the exact warning from bug: buildEs5 emits warning with Vite #4122, and the output built with this fix produces no warning.

Other information

esm-es5 output before this change:

return import("./".concat(o,".entry.js").concat("")).then(...)

[johnjenkins]

hey again @Arul1998 - thanks for looking into this, but ES5 builds are being dropped in the next release of Stencil so this work won't be necessary.

[johnjenkins]

Edit ... after looking, I changed my mind as it's such a small change 😅