Typeguard fails in function after instanceof

[hpop]

TypeScript Version: nightly (2.0.0-dev.201xxxxx)

Code

class A {
  foo = 'hello';
}

class B extends A {
  bar = 'hello';
}

let a:A = new B();

if (a instanceof B) {
  console.log(a.bar); // working
}

if (a instanceof B) {
  setTimeout( () => {
    console.log(a.bar); // error
  });
}

Expected behavior:
TypeScript should recognize a as instance of B. This worked in 1.8

Actual behavior:
test.ts(17,17): error TS2339: Property 'bar' does not exist on type 'A'.

[roganov]

Duplicate of #7719 (and others). Declaring const a:A = new B(); instead of let a:A = new B(); should help.