[PR] Improve FileReader types
[PR] Improve FileReader types

Summary:
Update the `FileReader` types to follow the [spec](https://w3c.github.io/FileAPI/#dfn-filereader) more closely.
Pull Request resolved: https://github.com/facebook/flow/pull/7952

Reviewed By: gkz

Differential Revision: D16426656

fbshipit-source-id: cf0c31c5c123abeac266137123022ef670c75cb0

nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow