Reflect.has fails to act as type guard (should act same as "in" operator)

**TypeScript Version:** ^3.4.0-dev.20190330 **Search Terms:** Reflect.has in operator **Code** ```ts const test1 = (a: { field: number } | {}) => (("field" in a) ? a.field : 0); const test2 = (a: { field: number } | {}) => Reflect.has(a, "field") ? a.field : 0; ``` **Expected behavior:** Both compile successfully. "in" operator acts as a type guard per https://github.com/Microsoft/TypeScript/issues/10485. Reflect.has should act [the same as "in" operator here](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Reflect/has). We'd like to use Reflect.has a lot more and this case holds us back. Thank you! **Actual behavior:** "in" operator line compiles; `Reflect.has` line does not compile. Error: > allPasos.ts:445:31 - error TS2339: Property 'field' does not exist on type '{} | { field: number; }'. > Property 'field' does not exist on type '{}'. 445 Reflect.has(a, "field") ? a.field : 0; **Playground Link:** https://goo.gl/2R4dkS **Related Issues:** https://github.com/Microsoft/TypeScript/issues/10485