ESLint: Enable flowtype/no-existential-type rule (#2032)

Author: IvanGoncharov

.eslintrc.yml

@@ -20,7 +20,7 @@ rules:
   flowtype/define-flow-type: error
   flowtype/newline-after-flow-annotation: [error, always]
   flowtype/no-dupe-keys: error
-  flowtype/no-existential-type: off # TODO
+  flowtype/no-existential-type: error
   flowtype/no-flow-fix-me-comments: off # TODO
   flowtype/no-mixed: off
   flowtype/no-mutable-array: off

src/type/definition.js

@@ -1,5 +1,8 @@
 // @flow strict
 
+// FIXME
+/* eslint-disable flowtype/no-existential-type */
+
 import objectEntries from '../polyfills/objectEntries';
 import defineToJSON from '../jsutils/defineToJSON';
 import defineToStringTag from '../jsutils/defineToStringTag';