Warn if properties not initialized in constructor

[Example](http://tryflow.org/#f53da3ebbf00a0e4165231d3d59c22ae):

``` es6
class C {
x: number;
y: number;
constructor() {
this.x = 0;
}
}

new C().y.toString(); // should produce error
```

I know this is complicated (what about derived classes? what if there is `this.initializeY()`?) but I don't think I've seen it being discussed on the issue tracker so far...