Request: Class Decorator Mutation

If we can get this to type check properly, we would have perfect support for boilerplate-free mixins:

``` typescript
declare function Blah<T>(target: T): T & {foo: number}

@Blah
class Foo {
    bar() {
        return this.foo; // Property 'foo' does not exist on type 'Foo'
    }
}

new Foo().foo; // Property 'foo' does not exist on type 'Foo'
```
