Merging ambient class and interface declaration

The instance side of an ambient class declaration can be extended using an interface declaration The class constructor object is unmodified. For example:

declare class Foo {
    public x : number;
}
interface Foo {
    y : string;
}
function bar(foo : Foo)  {
    foo.x = 1; // OK, declared in the class Foo
    foo.y = "1"; // OK, declared in the interface Foo
}