Objects with readonly fields can assigned to objects without having readonly modifier

[cevek]

TypeScript Version: 2.4.0-dev.20170519

Code

interface FooReadonly {
    readonly foo: number;
}

interface Foo {
    foo: number;
}
let y:FooReadonly = {foo: 1};
let x:Foo = y;
x.foo = 2;

Expected behavior:
Should throw an error

Actual behavior:
No error

[ghost]

Duplicate of #13347

[mhegazy]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.