fix(comments): preserve comments

[Kristinita]

1. Summary

eslint-plugin-prefer-arrow-functions with my changes preserves Flow comment types and their formatting.

My changes should fix the issue #24.

2. The reason for the need for the feature

Before my changes, eslint-plugin-prefer-arrow-functions removed all Flow comment types if a user ran ESLint with the argument --fix. It’s the critical problem for users using Flow comment-based syntax. Flow checks types inside comments.

I tested my changes in my real project with Flow comment types, and I can’t find cases of unexpected behavior.

3. Notes

1. Not solely Flow users may need to preserve comments. I don’t know all the ways users add comments. Additional pull requests may be required.

2. The plugin doesn’t preserve formatting if a user has a multiline comment before a fat arrow. In CoffeeScript, I use a syntax like this:

   loadPreviousMetadata = (metadataFilePath ###: string ###) ###:
     {[key: string]: {files: {[key: string]: number, timestamp: number}}}
   ### ->
     # Function body

   JavaScript doesn’t support this syntax:

   loadPreviousMetadata = (metadataFilePath/*: string */)/*:
   	{[key: string]: {files: {[key: string]: number, timestamp: number}}}
   */ => {
     // Function body
   };

   To preserve code validity I couldn’t think of anything better than to convert multiline comments to single-line ones:

   loadPreviousMetadata = (metadataFilePath/*: string */)/*: {[key: string]: {files: {[key: string]: number, timestamp: number}}} */ => {
     // Function body
   };

Thanks.