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_array_spec.ts

@@ -622,6 +622,36 @@ import {of } from 'rxjs/observable/of';
         expect(c.pending).toEqual(true);
         expect(a.pending).toEqual(false);
       });
+
+      describe('status change events', () => {
+        let logger: string[];
+
+        beforeEach(() => {
+          logger = [];
+          a.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 from parent when onlySelf is true', () => {
+          c.markAsPending({onlySelf: true});
+          expect(logger).toEqual([]);
+        });
+
+        it('should not emit event when emitEvent = false', () => {
+          c.markAsPending({emitEvent: false});
+          expect(logger).toEqual([]);
+        });
+
+        it('should emit event when parent is markedAsPending', () => {
+          a.markAsPending();
+          expect(logger).toEqual(['PENDING']);
+        });
+      });
+
     });
 
     describe('valueChanges', () => {