refactor(forms): eagerly initialize data members (#44292)

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 #44292

packages/forms/src/model.ts

@@ -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>;
 
   /**