Observe non direct properties

[matjaz]

For example

class VM {
  get isValid() {
    // var fullName = this.user.fullName
    return this.user.isValid();
  }
}
class User {
  isValid() {
    return this.fullName.length > 0;
  }
}

isValid is not updated. But if we uncomment var fullName it is.

I suggest adding something like

@computedFrom('user.fullName')
get isValid() {
  return this.user.isValid();
}

[jdanyow]

tracking this one here: aurelia/binding#149