Type Guard on Field does not Narrow Type of Object

[AnyhowStep]

TypeScript Version: 3.0.0-dev.20180602

Search Terms: field type guard narrow

Code

interface A {
  f?: number;
  bar: string; 
}

interface B {
  f: number; 
}
declare const x: A | B;
if (x.f == undefined) {
  //Expected:
  //'x' is narrowed to type 'A'
  //Actual:
  //Property 'bar' does not exist on type 'A | B'.
  //Property 'bar' does not exist on type 'B'.
  console.log(x.bar);
} 

Expected behavior:

'x' is narrowed to type 'A'

Actual behavior:

Property 'bar' does not exist on type 'A | B'.
Property 'bar' does not exist on type 'B'.

Playground Link: Here

[mhegazy]

This is in the vein of #24193, with undefined instead of null. I would recommend adding this use case to #24193