In JS, this assignments in a function add undefined to the resulting property type

This is visible with `--strict`:

```ts
// @noEmit: true
// @allowJs: true
// @checkJs: true
// @strict: true
// @Filename: a.js

function A() {
    this.x = { y : 1 }
}
var a = new A()
a.x.y
```

**Expected behavior:**
No error

**Actual behavior:**
Error "Object is possibly undefined" when accessing `a.x.y`