Declare the dataAdapter since it will be available at runtime if some…

…one overrides it

types/adapters.d.ts

@@ -58,6 +58,11 @@ export interface DateAdapter {
 	endOf(timestamp: number, unit: TimeUnit | 'isoWeek'): number;
 }
 
+declare const DateAdapter: {
+	prototype: DateAdapter;
+	new(options: any): DateAdapter;
+};
+
 export const _adapters: {
 	_date: DateAdapter;
 };