app.component.ts

function conditionalValidator(predicate, validator) {
  return (formControl => {
    if (!formControl.parent) {
      return null;
    }
    if (predicate()) {
      return validator(formControl); 
    }
    return null;
  })
}
//in ngOnInit:
this.myForm = this.fb.group({
  myCheckbox: [''],
  myEmailField: ['', [
     Validators.maxLength(250),
     Validators.minLength(5),
     Validators.pattern(/.+@.+\..+/),
     conditionalValidator(() => this.myForm.get('myCheckbox').value, 
                          pattern(/.+@.+\..+/),)
  ]]
})