Impossible implement another class with private fields

[cevek]

TypeScript Version: 2.5.0 nightly

Code

class Foo {
    baz: string;
    private foo: string; 
}

//TS2420:Class 'Bar' incorrectly implements interface 'Foo'.
//  Property 'foo' is missing in type 'Bar'.
class Bar implements Foo {
    baz: string;
}

But if add private member foo to Bar it throws
TS2420:Class 'Bar' incorrectly implements interface 'Foo'.
Types have separate declarations of a private property 'foo'.

[RyanCavanaugh]

See #471