feat(forms): add easy way to switch between forms modules (#9202)

modules/@angular/common/src/forms-deprecated/directives/ng_form.ts

@@ -95,6 +95,11 @@ export class NgForm extends ControlContainer implements Form {
       @Optional() @Self() @Inject(NG_VALIDATORS) validators: any[],
       @Optional() @Self() @Inject(NG_ASYNC_VALIDATORS) asyncValidators: any[]) {
     super();
+    console.warn(`
+      *It looks like you're using the old forms module. This will be opt-in in the next RC, and
+      will eventually be removed in favor of the new forms module. For more information, see:
+      https://docs.google.com/document/u/1/d/1RIezQqE4aEhBRmArIAS1mRIZtWFf6JxN_7B4meyWK0Y/pub
+    `);
     this.form = new ControlGroup(
         {}, null, composeValidators(validators), composeAsyncValidators(asyncValidators));
   }

modules/@angular/common/src/forms-deprecated/directives/ng_form_model.ts

@@ -113,6 +113,11 @@ export class NgFormModel extends ControlContainer implements Form,
       @Optional() @Self() @Inject(NG_VALIDATORS) private _validators: any[],
       @Optional() @Self() @Inject(NG_ASYNC_VALIDATORS) private _asyncValidators: any[]) {
     super();
+    console.warn(`
+      *It looks like you're using the old forms module. This will be opt-in in the next RC, and
+      will eventually be removed in favor of the new forms module. For more information, see:
+      https://docs.google.com/document/u/1/d/1RIezQqE4aEhBRmArIAS1mRIZtWFf6JxN_7B4meyWK0Y/pub
+    `);
   }
 
   ngOnChanges(changes: SimpleChanges): void {