fix(forms): allow arrays as parents (#10440)

Closes #10432

modules/@angular/forms/src/directives/reactive_errors.ts

@@ -7,6 +7,7 @@
  */
 
 import {BaseException} from '../facade/exceptions';
+
 import {FormErrorExamples as Examples} from './error_examples';
 
 export class ReactiveErrors {
@@ -60,4 +61,4 @@ export class ReactiveErrors {
 
         ${Examples.formArrayName}`);
   }
-}
+}