Enable Flow Strict Local for 13 files in xplat/sonar

Summary:
First add a Flow Strict config, then enable Flow Strict Local for 13 files in xplat/sonar.

each change can land seperately:
drop-conflicts

Reviewed By: passy

Differential Revision: D13368791

fbshipit-source-id: 5bc1f9d3a179f4b9a554cc8cc629db0fda468a36

static/compilePlugins.js

@@ -3,7 +3,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  * @format
- * @flow
+ * @flow strict-local
  */
 
 const path = require('path');