javascript ES6 promise is not executed in serial order

promise-is-non-serial.es.js

var flag = false;
var promise = Promise.resolve();

// not linear execution of code! 
// is the then callback pushed onto the call-stack. how ?
promise.then( (x) => {
  console.log(`flag is ${flag}`); // flag is true
})

flag = true

noob code to shim a promise. does not work

class DummyPromise {
  then(x) {
    x.call();
  }
}
var flag = false;
var stub = new DummyPromise();

() => {
  () => {
    console.log(`flag is ${flag}`);  
  }()
  console.log(`flag is ${flag}`);  
}()

stub.then(() => {
  console.log(`flag is ${flag}`);
})

flag = true