Support method unbound for fs/promises in flow 0.154

Summary:
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This PR allows using fs/promises like this in flow 0.154:
```js
const {writeFile} = require('fs').promises;
```

Node fs/promises doc: https://nodejs.org/api/fs.html#fs_promises_api

Pull Request resolved: #8697

Reviewed By: dsainati1

Differential Revision: D29451697

Pulled By: gkz

fbshipit-source-id: 3f0638503681c1f69f82db17a1ac661a55f30405

lib/node.js

@@ -1321,7 +1321,7 @@ declare module "fs" {
   }
 
   declare type FSPromisePath = string | Buffer | URL;
-  declare class FSPromise {
+  declare type FSPromise = {
     access(path: FSPromisePath, mode?: number): Promise<void>,
     appendFile(path: FSPromisePath | FileHandle, data: string | Buffer, options: WriteOptions | string): Promise<void>,
     chmod(path: FSPromisePath, mode: number): Promise<void>,
@@ -1356,12 +1356,15 @@ declare module "fs" {
       ...
     }>,
     readdir(path: FSPromisePath, options?: string | EncodingOptions): Promise<string[]>,
-    readFile(path: FSPromisePath | FileHandle, options: string): Promise<string>,
-    readFile(path: FSPromisePath | FileHandle, options?: EncodingFlag): Promise<Buffer>,
-    readlink(path: FSPromisePath, options: BufferEncoding): Promise<Buffer>,
-    readlink(path: FSPromisePath, options?: string | EncodingOptions): Promise<string>,
-    realpath(path: FSPromisePath, options: BufferEncoding): Promise<Buffer>,
-    realpath(path: FSPromisePath, options?: string | EncodingOptions): Promise<string>,
+    readFile:
+      & ((path: FSPromisePath | FileHandle, options: string) => Promise<string>)
+      & ((path: FSPromisePath | FileHandle, options?: EncodingFlag) => Promise<Buffer>),
+    readlink:
+      & ((path: FSPromisePath, options: BufferEncoding) => Promise<Buffer>)
+      & ((path: FSPromisePath, options?: string | EncodingOptions) => Promise<string>),
+    realpath:
+      & ((path: FSPromisePath, options: BufferEncoding) => Promise<Buffer>)
+      & ((path: FSPromisePath, options?: string | EncodingOptions) => Promise<string>),
     rename(oldPath: FSPromisePath, newPath: FSPromisePath): Promise<void>,
     rmdir(path: FSPromisePath): Promise<void>,
     stat(path: FSPromisePath): Promise<Stats>,
@@ -1385,6 +1388,7 @@ declare module "fs" {
       data: string | Buffer | Uint8Array,
       options?: string | WriteOptions
     ): Promise<void>,
+    ...
   }
 
   declare var promises: FSPromise;