FormControls, FormGroups, and FormArrays now optionally accept an options
object as their second argument. Validators and async validators can be
passed in as part of this options object (though they can still be passed
in as the second and third arg as before).
```ts
const c = new FormControl(, {
validators: [Validators.required],
asyncValidators: [myAsyncValidator]
});
```
This commit also adds support for passing arrays of validators and async
validators to FormGroups and FormArrays, which formerly only accepted
individual functions.
```ts
const g = new FormGroup({
one: new FormControl()
}, [myPasswordValidator, myOtherValidator]);
```
This change paves the way for adding more options to AbstractControls,
such as more fine-grained control of validation timing.
Observable subscriptions from previous validation runs should be canceled
before a new subscription is created for the next validation run.
Currently the subscription that sets the errors is canceled properly,
but the source observable created by the validator is not. While this
does not affect validation status or error setting, the source
observables will incorrectly continue through the pipeline until they
complete. This change ensures that the whole stream is canceled.
AsyncValidatorFn previously had an "any" return type, but now it more
explicitly requires a Promise or Observable return type. We don't
anticipate this causing problems given that any other return type
would have caused a runtime error already.
