No compile error thrown when `this` referenced before call to `super` completes

**TypeScript Version:** 1.8.9 **Code** JSFiddle: https://jsfiddle.net/kitsonk/fs9t96ep/ TS Playground: http://goo.gl/X7cgvV ``` ts class A { constructor(fn: () => void) { fn.call(this); } foo: string = 'foo'; } class B extends A { constructor() { super(() => { console.log(this); }); } bar: string = 'bar'; } const b = new B(); ``` **Expected behavior:** Typescript should guard against the use of `this` as the call to `super` has not completed. Thus it should not compile. **Actual behavior:** Typescript compiles this successfully. It works fine when the target is set to `es5` but breaks the browser when target is set to `es6`. Error is: VM89:55 Uncaught ReferenceError: this is not defined