__importStar sometimes inlined when using dynamic imports with --importHelpers

TypeScript Version: 3.1.1

Search Terms: __importStar tslib

Code
a.ts:

import('b').then(b => console.log(b));

compile with

tsc a.ts --esModuleInterop --importHelpers

Expected behavior:
some type errors, but emitted code should require("tslib") and use __importStar from there

Actual behavior:
some type errors and generated code looks like this:

var __importStar = (this && this.__importStar) || function (mod) {
    if (mod && mod.__esModule) return mod;
    var result = {};
    if (mod != null) for (var k in mod) if (Object.hasOwnProperty.call(mod, k)) result[k] = mod[k];
    result["default"] = mod;
    return result;
};
Promise.resolve().then(function () { return __importStar(require('b')); }).then(function (b) { return console.log(b); });

btw an example that works correctly:

import('c').then(b => console.log(b));

class Foo {}

export class Bar extends Foo {}

both the extends and the export seem to be necessary

Playground Link: N/A (--esModuleInterop doesn't exist in the playground)

Related Issues: This is basically #21560 but replacing regular imports with dynamic imports

Reproduce el informe con a.ts usando tsc 3.1.1, --esModuleInterop y --importHelpers; después, inspecciona el código emitido para la importación dinámica. Traza cómo se selecciona __importStar para la importación basada en Promise y compáralo con el ejemplo funcional que incluye extends y export. Se considera terminado cuando el caso que falla requiere tslib y usa su helper __importStar en lugar de incluirlo inline.