Casting of a union type variable doesn't work if interface has all optional properties

TypeScript Version: at least 2.0.0 - 2.3.0

Code

interface ITest1 { IsTest: boolean; }
interface ITest2 { IsTest?: boolean; }

let case1: ITest1 | string;
case1 = case1 as string;
case1.indexOf("_"); // <-- no error

let case2: ITest2 | string;
case2 = case2 as string;
case2.indexOf("_"); // <-- ERROR

Expected behavior:
In the second case there should be no error reported, same as in the first one. case2 variable should have type string after casting case2 = case2 as string

Actual behavior:
When union of types contain interface with all optional properties than casting as one of the types does not change the type of a variable to more specific one.

Start by running the TypeScript reproduction shown in the issue and trace how a union containing an all-optional interface is handled after a type assertion. Add a regression test for the two cases, and consider the issue done when the second variable is accepted as a string and the existing first case remains correct.