Sets the `checked` attribute on the element, if the expression inside `ngChecked` is truthy.
Note that this directive should not be used together with `ngModel`,
as this can lead to unexpected behavior.
### Why do we need `ngChecked`?
The HTML specification does not require browsers to preserve the values of boolean attributes
such as checked. (Their presence means true and their absence means false.)
If we put an Angular interpolation expression into such an attribute then the
binding information would be lost when the browser removes the attribute.
The `ngChecked` directive solves this problem for the `checked` attribute.
This complementary directive is not removed by the browser and so provides
a permanent reliable place to store the binding information.