Implement the active-descendant a11y rule

src/compiler/compile/compiler_warnings.ts

@@ -187,6 +187,10 @@ export default {
 		code: 'a11y-no-noninteractive-tabindex',
 		message: 'A11y: noninteractive element cannot have nonnegative tabIndex value'
 	},
+	a11y_aria_activedescendant_has_tabindex: {
+		code: 'a11y-aria-activedescendant-has-tabindex',
+		message: 'A11y: Elements with attribute aria-activedescendant should have tabindex value'
+	},
 	redundant_event_modifier_for_touch: {
 		code: 'redundant-event-modifier',
 		message: 'Touch event handlers that don\'t use the \'event\' object are passive by default'

src/compiler/compile/nodes/Element.ts

@@ -484,6 +484,11 @@ export default class Element extends Node {
 						component.warn(attribute, compiler_warnings.a11y_incorrect_attribute_type(schema, name));
 					}
 				}
+
+				// aria-activedescendant-has-tabindex
+				if (name === 'aria-activedescendant' && !is_interactive_element(this.name, attribute_map) && !attribute_map.has('tabindex')) {
+			    component.warn(attribute, compiler_warnings.a11y_aria_activedescendant_has_tabindex);
+				}
 			}
 
 			// aria-role