message: refactor(forms): eagerly initialize data members
  Data members in AbstractControl should be eagerly
  initialized to address issue/24571. This eliminates the need to
  constantly check for truthiness and makes code much more readable.
  More PRs to follow to address issue/24571.
  PR Close
diff: @@ -168,8 +168,7 @@ function isOptionsObj(validatorOrOpts?: ValidatorFn|ValidatorFn[]|AbstractContro
  */
  export abstract class AbstractControl {
  /** @internal */
  - // TODO(issue/24571): remove '!'.
  - _pendingDirty!: boolean;
  + _pendingDirty = false;
  /**
  * Indicates that a control has its own pending asynchronous validation in progress.
  @@ -179,15 +178,13 @@ export abstract class AbstractControl {
  _hasOwnPendingAsyncValidator = false;
  /** @internal */
  - // TODO(issue/24571): remove '!'.
  - _pendingTouched!: boolean;
  + _pendingTouched = false;
  /** @internal */
  _onCollectionChange = () => {};
  /** @internal */
  - // TODO(issue/24571): remove '!'.
  - _updateOn!: FormHooks;
  + _updateOn?: FormHooks;
  private _parent: FormGroup|FormArray|null = null;
  private _asyncValidationSubscription: any;
  @@ -298,7 +295,6 @@ export abstract class AbstractControl {
  * These status values are mutually exclusive, so a control cannot be
  * both valid AND invalid or invalid AND disabled.
  */
  - // TODO(issue/24571): remove '!'.
  public readonly status!: FormControlStatus;
  /**
  @@ -369,7 +365,6 @@ export abstract class AbstractControl {
  * An object containing any errors generated by failing validation,
  * or null if there are no errors.
  */
  - // TODO(issue/24571): remove '!'.
  public readonly errors!: ValidationErrors|null;
  /**
  @@ -415,7 +410,6 @@ export abstract class AbstractControl {
  * the UI or programmatically. It also emits an event each time you call enable() or disable()
  * without passing along {emitEvent: false} as a function argument.
  */
  - // TODO(issue/24571): remove '!'.
  public readonly valueChanges!: Observable<any>;
  /**
  @@ -426,7 +420,6 @@ export abstract class AbstractControl {
  * @see {@link AbstractControl.status}
  *
  */
  - // TODO(issue/24571): remove '!'.
  public readonly statusChanges!: Observable<FormControlStatus>;
  /**
  