Apply feedback for RegExp.prototype.flags (#1196)

test/built-ins/RegExp/prototype/flags/coercion-dotall.js

test/built-ins/RegExp/prototype/flags/coercion-global.js

test/built-ins/RegExp/prototype/flags/coercion-ignoreCase.js

test/built-ins/RegExp/prototype/flags/coercion-multiline.js

test/built-ins/RegExp/prototype/flags/coercion-sticky.js

test/built-ins/RegExp/prototype/flags/coercion-unicode.js

test/built-ins/RegExp/prototype/flags/coercion.js

test/built-ins/RegExp/prototype/flags/get-order.js

test/built-ins/RegExp/prototype/flags/rethrow.js

test/built-ins/RegExp/prototype/flags/return-order.js

...t-ins/RegExp/prototype/flags/this-val-regexp-prototype.js

test/built-ins/RegExp/prototype/flags/this-val-regexp.js