fix(compiler): eliminate false-positive mixin warnings for barrel imports and useDefineForClassFields:false

[davidpett]

Summary

Two separate conditions in mergeExtendedClassMeta / resolveAndProcessExtendedClass produced spurious build warnings for valid mixin-factory patterns. Both are fixed in a single commit.

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Fix 1 — useDefineForClassFields: false false positive

Root cause: detectModernPropDeclarations returns false whenever TypeScript has stripped PropertyDeclaration nodes from the class body. With useDefineForClassFields: false that stripping always happens, so the guard fired unconditionally and emitted:

"Component classes can only extend from other Stencil decorated base classes when targeting more modern JavaScript"

Fix: Guard the warning with buildCtx.config.tsCompilerOptions?.useDefineForClassFields !== false so the check is skipped entirely in that configuration (which is the recommended Stencil setup for "target": "es2022").

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Fix 2 — Barrel re-export support

Root cause: When a mixin factory is imported through a barrel index file:

// mixins/index.ts
export * from './color';

// component.tsx
import { colorFactory } from '../mixins';

the direct statements.find() lookup in resolveAndProcessExtendedClass returns null because the barrel has no own declarations. This triggered:

"Please import class / mixin-factory declarations directly and not via barrel files"

even though the pattern is completely valid.

Fix: Added a resolveFromBarrelExports helper that follows export * from '...' and export { X } from '...' re-exports one level deep to locate the real declaration. The call site tries barrel traversal before emitting the warning. The dependentClasses entry is updated to record the actual source file / file name (not the barrel's) for correct downstream processing.

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Test infrastructure

File	What was added
src/compiler/transformers/test/transpile.ts	transpileModules(files, config?, tsConfig?) — multi-file variant of transpileModule that pre-populates compilerCtx.moduleMap with auxiliary source files so cross-file scenarios work in unit tests without hitting the real file system
src/compiler/transformers/test/parse-mixin.spec.ts	describe('mixin factory imported from a barrel index file') with 3 test cases: export * from barrel, named export { X as Y } from barrel, and explicit useDefineForClassFields: false

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Testing

• All 17 pre-existing tests in the relevant spec files continue to pass.
• The 3 new tests pass.
• npm run ts scripts/index.ts -- --prod --ci completes successfully. The only tsc.prod errors are pre-existing in src/testing/jest/jest-28/jest-environment.ts (unrelated, present on main).
• Verified end-to-end in a downstream project that uses the mixin factory + barrel pattern with "useDefineForClassFields": false, "target": "es2022" — zero warnings after applying the fix.