Elide in/out variance modifiers from emitted JS

[Copilot]

in/out variance modifiers are only valid on type parameters, but when written (a grammar error) on a class member they were copied verbatim into the emitted JavaScript:

class C {
  in x = 1;
}

// before          // after
class C {          class C {
    in x = 1;          x = 1;
}                  }

Changes

• internal/transformers/tstransforms/typeeraser.go: TypeEraserTransformer now elides KindInKeyword/KindOutKeyword modifier tokens alongside the other TypeScript-only modifiers. Since KindInKeyword is shared with the in binary operator, the token is only elided when its parent is not a BinaryExpression, so a in b and for…in are left untouched.
• testdata/tests/cases/compiler/varianceModifiersOnClassMembers.ts (+ baselines): covers variance modifiers on class members together with the in operator and for…in to guard against over-elision.

The grammar diagnostic (TS1274) is unaffected and still reported.

[Copilot]

Pull request overview

Fixes a regression where in/out variance modifiers (grammar errors when placed on class members rather than type parameters) were copied verbatim into emitted JavaScript. The TypeEraserTransformer now elides these tokens while preserving the in binary operator and for…in semantics.

Changes:

• Add an InKeyword/OutKeyword case in TypeEraserTransformer.visit that elides the token unless its parent is a BinaryExpression.
• Add a compiler test case (with .js, .types, .symbols, .errors.txt baselines) covering misplaced variance modifiers alongside in/for…in usage.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

File	Description
internal/transformers/tstransforms/typeeraser.go	Elides KindInKeyword/KindOutKeyword modifier tokens, guarded against the in binary operator via parent-node check.
testdata/tests/cases/compiler/varianceModifiersOnClassMembers.ts	New test covering elision plus in operator/for…in preservation.
testdata/baselines/reference/compiler/varianceModifiersOnClassMembers.js	Baseline showing variance modifiers stripped, in/for…in retained.
testdata/baselines/reference/compiler/varianceModifiersOnClassMembers.{types,symbols,errors.txt}	Type/symbol/diagnostic baselines (TS1274 still reported).

[RyanCavanaugh]

Seems fine but I don't think we guarantee syntactic validity of output JS in the presence of malformed TS, which this is? I would have punted the original bug

Same category as

const q = ;

which emits as-is (syntax error in, syntax error out)

[jakebailey]

A while back I added something to our test harness that verified that all output code parses, and it does; I think surprisingly it was an invariant, we just were lacking many tests.

[RyanCavanaugh]

Yeah seems like a desirable property to have, even if Strada didn't have it