Type guards in string literals in callbacks

[normalser]

TypeScript Version: (2.0.0-beta)

Code

interface Test1 {
    type: 'test1'
    label: 'string'
}
interface Test2 {
    type: 'test2'
}

let a: Test1 | Test2

if (a.type == 'test1') {
    console.log(a.label) // works ok
    setTimeout(()=>{
        console.log(a.label) // Error: Property 'label' does not exist on type 'Test1 | Test2'
    })
}

Expected behavior:
setTimeout case should work ?

Actual behavior:
Error: Property 'label' does not exist on type 'Test1 | Test2' in setTimeout

[mhegazy]

Duplicate of #7662. please see #7719 (comment) for more details.