use `onlyStrict` flag instead of `"use strict"` directive

test/built-ins/Array/prototype/flatMap/thisArg-argument.js

@@ -6,10 +6,9 @@ description: >
     Behavior when thisArg is provided
     Array.prototype.flatMap ( mapperFunction [ , thisArg ] )
 includes: [compareArray.js]
+flags: [onlyStrict]
 ---*/
 
-"use strict";
-
 var a = {};
 
 assert(compareArray([1].flatMap(function() { return [this]; }, "TestString"), ["TestString"]));