fix(parser): inject params into one-line non-async arrow scenarios

[mirao]

Fixes #5679

Problem

Scenario('...', ({ I }) => { if (true) { I.say('hello') } }) injects nothing — I, page objects and current are all undefined, and the test dies with TypeError: Cannot read properties of undefined.

parse-function@5.6.10 decides whether its input is an ES6 object method with:

const isMethod = /^\*?.+\([\S\W]*\)\s*{/i.test(result.value)
if (!(isFunction || isAsyncFn || isAsyncArrow) && isMethod) result.value = `{ ${result.value} }`

The greedy .+ combined with [\S\W]* matches any source containing a ) { sequence, so a non-async arrow whose body holds an if, for, while or switch is wrapped in braces as a fake object method and acorn throws. getParams() logs the SyntaxError and returns undefined.

Async arrows escape via the library's own isAsyncArrow check. In plain JavaScript a multi-line arrow escapes too, because . does not cross a newline. That second escape does not exist under TypeScript — tsx/esbuild emit every function on one line, so fn.toString() returns the one-line form however the source was written. In a TypeScript suite every non-async scenario with destructured params and a conditional fails, which is why this surfaced as a 4.0 regression.

Fix

normalizeArrowFn() asks acorn whether the source really is an ArrowFunctionExpression and, if so, hands parse-function async <source> so it takes the isAsyncArrow branch instead of the method-wrapping one.

Anything acorn does not confirm as an arrow — class methods, generators, function expressions, strings, unparseable input — is returned untouched, so only input that already fails is affected. The prefix cannot change a parameter list, and default values stay correct because their offsets are sliced from the same prefixed string.

ecmaVersion becomes a shared const so the parse and the arrow check cannot drift apart. Its value is unchanged, so no syntax gains or loses parseability.

Verification

Check	Result
New tests with lib/parser.js reverted	4 failing
New tests with the fix	12 passing (8 pre-existing + 4 new)
Differential vs. the old parser, 47 function shapes, both getParams and getParamsToString	0 regressions, 15 fixes
npm run test:unit	773 passed, 0 failed
npm run test:runner	276 passed, 0 failed
Repro from the issue, incl. Data().Scenario(({ current }) => ...)	4 passed, params injected

The differential corpus covered class methods, async/static methods, getters, generators and async generators, function expressions, expression-body arrows, rest params, default values (numeric, string, call expressions), nested arrows, strings containing =>, sinon proxies, string inputs and garbage input.

Not included

A one-line arrow whose body uses ES2021+ syntax (??=, ||=, numeric separators, class fields) still gets no params, because ecmaVersion: 11 pins the parser to ES2020. That is a separate, pre-existing defect with the same symptom — unrelated to the isMethod misdetection and unchanged by this PR. Raising it to 'latest' fixes it and was green on both suites, but it is deliberately left out to keep this PR to the reported bug. Happy to add it here or in a follow-up if maintainers prefer.

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