Annotations for the File System API (#9127)

Summary:
This PR adds flow library definitions for the [File System API](https://developer.mozilla.org/en-US/docs/Web/API/File_System_API)

I've only added methods and functions that are part of the [WHATWG Standard](https://fs.spec.whatwg.org/#introduction).

Furthermore, I had to add missing flow library definitions for `WritableStream` and `WritableStreamDefaultWriter`, which are a part of the [Streams API](https://streams.spec.whatwg.org/#default-writer-class), since these are a dependency of `FileSystemWritableFileStream`.

API References:
- `FileSystemHandle`: [MDN](https://developer.mozilla.org/en-US/docs/Web/API/FileSystemHandle), [WHATWG](https://fs.spec.whatwg.org/#api-filesystemhandle)
- `FileSystemFileHandle`: [MDN](https://developer.mozilla.org/en-US/docs/Web/API/FileSystemFileHandle), [WHATWG](https://fs.spec.whatwg.org/#api-filesystemfilehandle)
- `FileSystemDirectoryHandle`: [MDN](https://developer.mozilla.org/en-US/docs/Web/API/FileSystemDirectoryHandle), [WHATWG](https://fs.spec.whatwg.org/#api-filesystemdirectoryhandle)
- `FileSystemSyncAccessHandle`: [MDN](https://developer.mozilla.org/en-US/docs/Web/API/FileSystemSyncAccessHandle), [WHATWG](https://fs.spec.whatwg.org/#api-filesystemsyncaccesshandle)
- `FileSystemWritableFileStream`: [MDN](https://developer.mozilla.org/en-US/docs/Web/API/FileSystemWritableFileStream), [WHATWG](https://fs.spec.whatwg.org/#api-filesystemwritablefilestream)

Pull Request resolved: #9127

Reviewed By: skyout

Differential Revision: D54821048

Pulled By: ZelJin

fbshipit-source-id: 7a61967e3ac13f0c566971b4f790ced076a3ce8a

lib/bom.js

@@ -160,6 +160,10 @@ type USBPermissionDescriptor = {|
   exclusionFilters: Array<USBDeviceFilter>;
 |}
 
+type FileSystemHandlePermissionDescriptor = {|
+  mode: "read" | "readwrite";
+|}
+
 declare class PermissionStatus extends EventTarget {
   onchange: ?((event: any) => mixed);
   +state: PermissionState;
@@ -2441,3 +2445,124 @@ type SetUpOptions = {
   index: number;
   ...
 }
+
+declare type FileSystemHandleKind = "file" | "directory";
+
+// https://wicg.github.io/file-system-access/#api-filesystemhandle
+declare class FileSystemHandle {
+  +kind: FileSystemHandleKind;
+  +name: string;
+
+  isSameEntry: (other: FileSystemHandle) => Promise<boolean>;
+  queryPermission?: (
+    descriptor: FileSystemHandlePermissionDescriptor
+  ) => Promise<PermissionStatus>;
+  requestPermission?: (
+    descriptor: FileSystemHandlePermissionDescriptor
+  ) => Promise<PermissionStatus>;
+}
+
+// https://fs.spec.whatwg.org/#api-filesystemfilehandle
+declare class FileSystemFileHandle extends FileSystemHandle {
+  +kind: "file";
+
+  constructor(name: string): void;
+
+  getFile(): Promise<File>;
+  createSyncAccessHandle(): Promise<FileSystemSyncAccessHandle>;
+  createWritable(options?: {|
+    keepExistingData?: boolean,
+  |}): Promise<FileSystemWritableFileStream>;
+}
+
+// https://fs.spec.whatwg.org/#api-filesystemdirectoryhandle
+declare class FileSystemDirectoryHandle extends FileSystemHandle {
+  +kind: "directory";
+
+  constructor(name: string): void;
+
+  getDirectoryHandle(
+    name: string,
+    options?: {| create?: boolean |}
+  ): Promise<FileSystemDirectoryHandle>;
+  getFileHandle(
+    name: string,
+    options?: {| create?: boolean |}
+  ): Promise<FileSystemFileHandle>;
+  removeEntry(name: string, options?: {| recursive?: boolean |}): Promise<void>;
+  resolve(possibleDescendant: FileSystemHandle): Promise<Array<string> | null>;
+
+  // Async iterator functions
+  @@asyncIterator(): AsyncIterator<[string, FileSystemHandle]>;
+  entries(): AsyncIterator<[string, FileSystemHandle]>;
+  keys(): AsyncIterator<string>;
+  values(): AsyncIterator<FileSystemHandle>;
+}
+
+// https://fs.spec.whatwg.org/#api-filesystemsyncaccesshandle
+declare class FileSystemSyncAccessHandle {
+  close(): void;
+  flush(): void;
+  getSize(): number;
+  read(buffer: ArrayBuffer, options?: {| at: number |}): number;
+  truncate(newSize: number): void;
+  write(buffer: ArrayBuffer, options?: {| at: number |}): number;
+}
+
+// https://streams.spec.whatwg.org/#default-writer-class
+declare class WritableStreamDefaultWriter {
+  +closed: Promise<void>;
+  +desiredSize: number;
+  +ready: Promise<void>;
+
+  constructor(): void;
+
+  abort(reason?: string): Promise<void>;
+  close(): Promise<void>;
+  releaseLock(): void;
+  write(chunk: any): Promise<void>;
+}
+
+// https://streams.spec.whatwg.org/#ws-class
+declare class WriteableStream {
+  +locked: boolean;
+
+  constructor(): void;
+
+  abort(reason: string): Promise<string>;
+  close(): Promise<void>;
+  getWriter(): WritableStreamDefaultWriter;
+}
+
+// https://fs.spec.whatwg.org/#dictdef-writeparams
+declare type FileSystemWriteableFileStreamDataTypes =
+  | ArrayBuffer
+  | $TypedArray
+  | DataView
+  | Blob
+  | string;
+
+// https://fs.spec.whatwg.org/#dictdef-writeparams
+declare type FileSystemWriteableFileStreamData =
+  | FileSystemWriteableFileStreamDataTypes
+  | {|
+      type: "write",
+      position?: number,
+      data: FileSystemWriteableFileStreamDataTypes,
+    |}
+  | {|
+      type: "seek",
+      position: number,
+      data: FileSystemWriteableFileStreamDataTypes,
+    |}
+  | {|
+      type: "size",
+      size: number,
+    |};
+
+// https://fs.spec.whatwg.org/#api-filesystemwritablefilestream
+declare class FileSystemWritableFileStream extends WriteableStream {
+  write(data: FileSystemWriteableFileStreamData): Promise<void>;
+  truncate(size: number): Promise<void>;
+  seek(position: number): Promise<void>;
+}