Bug Fixes
- core: ensure validators key is defined on build (#2820) (51300c8), closes #2818
- json-schema: do not override widget.formlyConfig.key (#2807) (69140f0)
- json-schema: resolve recursive schema when using widget (#2827) (055c268), closes #2823
- json-schema: take account of optional object with required props (#2817) (6a97932), closes #2752
- material: allow passing aria-label to mat-select (#2828) (f26d7e2), closes #2821