The optional this should be readonly too.

src/lib/es2015.core.d.ts

@@ -353,7 +353,7 @@ interface ReadonlyArray<T> {
     * @param thisArg If provided, it will be used as the this value for each invocation of
     * predicate. If it is not provided, undefined is used instead.
     */
-  find(predicate: (value: T, index: number, obj: Array<T>) => boolean, thisArg?: any): T | undefined;
+  find(predicate: (value: T, index: number, obj: ReadonlyArray<T>) => boolean, thisArg?: any): T | undefined;
 
   /**
     * Returns the index of the first element in the array where predicate is true, and undefined