feat(forms): allow markAsPending to emit events (#20212)

closes #17958

BREAKING CHANGE:
- `AbstractControl#statusChanges` now emits an event of `'PENDING'` when you call `AbstractControl#markAsPending`
- Previously it did not emit an event when you called `markAsPending`
- To migrate you would need to ensure that if you are filtering or checking events from `statusChanges` that you account for the new event when calling `markAsPending`

PR Close #20212

packages/forms/test/form_control_spec.ts

@@ -1161,5 +1161,37 @@ import {FormArray} from '@angular/forms/src/model';
 
       });
     });
+    describe('pending', () => {
+      let c: FormControl;
+
+      beforeEach(() => { c = new FormControl('value'); });
+
+      it('should be false after creating a control', () => { expect(c.pending).toEqual(false); });
+
+      it('should be true after changing the value of the control', () => {
+        c.markAsPending();
+        expect(c.pending).toEqual(true);
+      });
+
+      describe('status change events', () => {
+        let logger: string[];
+
+        beforeEach(() => {
+          logger = [];
+          c.statusChanges.subscribe((status) => logger.push(status));
+        });
+
+        it('should emit event after marking control as pending', () => {
+          c.markAsPending();
+          expect(logger).toEqual(['PENDING']);
+        });
+
+        it('should not emit event when emitEvent = false', () => {
+          c.markAsPending({emitEvent: false});
+          expect(logger).toEqual([]);
+        });
+
+      });
+    });
   });
 })();