Add find and findIndex to ReadonlyArray

src/lib/es2015.core.d.ts

@@ -343,6 +343,30 @@ interface ObjectConstructor {
     defineProperty(o: any, propertyKey: PropertyKey, attributes: PropertyDescriptor): any;
 }
 
+interface ReadonlyArray<T> {
+  /**
+    * Returns the value of the first element in the array where predicate is true, and undefined
+    * otherwise.
+    * @param predicate find calls predicate once for each element of the array, in ascending
+    * order, until it finds one where predicate returns true. If such an element is found, find
+    * immediately returns that element value. Otherwise, find returns undefined.
+    * @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;
+
+  /**
+    * Returns the index of the first element in the array where predicate is true, and undefined
+    * otherwise.
+    * @param predicate find calls predicate once for each element of the array, in ascending
+    * order, until it finds one where predicate returns true. If such an element is found, 
+    * findIndex immediately returns that element index. Otherwise, findIndex returns -1.
+    * @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.
+    */
+  findIndex(predicate: (value: T) => boolean, thisArg?: any): number;
+}
+
 interface RegExp {
     /**
       * Returns a string indicating the flags of the regular expression in question. This field is read-only.