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[ x ] feature request
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Current behavior
I am extending the functionality from FormGroup with my own type. But I have some re-usability problems.

Right now the constructor at FormGroup is like:

constructor( public controls: {[key: string]: AbstractControl}, .....) {}

which does not allow to delegate functions like insert, get and iterate on controls to my derived class. Why not receiving an ES6 Map instead?

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map

Since "operators overloads" have been declined in Typescript (here), there is no nice way for me to, for instance, keep FormGroup children and FormControl on different objects.

I can submit a PR if you think it makes sense and I have a chance.