Define entry points for File System API (#9128)

Summary:
As a follow-up to #9127, this PR defines all currently available entry points for the [File System API](https://developer.mozilla.org/en-US/docs/Web/API/File_System_API):

- `window.showOpenFilePicker()`: [MDN](https://developer.mozilla.org/en-US/docs/Web/API/window/showOpenFilePicker), [WICG](https://wicg.github.io/file-system-access/#api-showopenfilepicker)
- `window.showSaveFilePicker()`: [MDN](https://developer.mozilla.org/en-US/docs/Web/API/Window/showSaveFilePicker), [WICG](https://wicg.github.io/file-system-access/#api-showsavefilepicker)
- `window.showDirectoryPicker()`: [MDN](https://developer.mozilla.org/en-US/docs/Web/API/window/showDirectoryPicker), [WICG](https://wicg.github.io/file-system-access/#api-showdirectorypicker)
- `DataTransferItem.getAsFileSystemHandle()`: [MDN](https://developer.mozilla.org/en-US/docs/Web/API/DataTransferItem/getAsFileSystemHandle), [WICG](https://wicg.github.io/file-system-access/#dom-datatransferitem-getasfilesystemhandle)
- `StorageManager.getDirectory()`: [MDN](https://developer.mozilla.org/en-US/docs/Web/API/StorageManager/getDirectory), [WHATWG](https://fs.spec.whatwg.org/#dom-storagemanager-getdirectory)

Pull Request resolved: #9128

Reviewed By: SamChou19815

Differential Revision: D54988446

Pulled By: ZelJin

fbshipit-source-id: 8758515aaf467f6c92c8a131436ea79000651c2b

lib/bom.js

@@ -288,6 +288,7 @@ declare class StorageManager {
   persist: () => Promise<boolean>;
   persisted: () => Promise<boolean>;
   estimate?: () => Promise<StorageEstimate>;
+  getDirectory: () => Promise<FileSystemDirectoryHandle>;
 }
 
 type StorageManagerRegisteredEndpoint = 'caches' | 'indexedDB' | 'localStorage' | 'serviceWorkerRegistrations' | 'sessionStorage';

lib/dom.js

@@ -113,6 +113,7 @@ declare class DataTransferItemList {
   clear(): void;
 };
 
+// https://wicg.github.io/file-system-access/#drag-and-drop
 declare class DataTransferItem {
   kind: string; // readonly
   type: string; // readonly
@@ -124,6 +125,12 @@ declare class DataTransferItem {
    * https://developer.mozilla.org/en-US/docs/Web/API/DataTransferItem/webkitGetAsEntry
    */
   webkitGetAsEntry(): void | () => any;
+  /*
+   * Not supported in all browsers
+   * For up to date compatibility information, please visit
+   * https://developer.mozilla.org/en-US/docs/Web/API/DataTransferItem/getAsFileSystemHandle
+   */
+  getAsFileSystemHandle?: () => Promise<?FileSystemHandle>;
 };
 
 /* DOM */
@@ -4522,6 +4529,67 @@ declare function scrollTo(options: ScrollToOptions): void;
 declare function scrollBy(x: number, y: number): void;
 declare function scrollBy(options: ScrollToOptions): void;
 
+/* Window file picker */
+
+type WindowFileSystemPickerFileType = {|
+  description?: string,
+  /*
+   * An Object with the keys set to the MIME type
+   * and the values an Array of file extensions
+   * Example:
+   * accept: {
+   *   "image/*": [".png", ".gif", ".jpeg", ".jpg"],
+   * },
+   */
+  accept: {
+    [string]: Array<string>,
+  },
+|};
+
+type WindowBaseFilePickerOptions = {|
+  id?: number,
+  startIn?:
+    | FileSystemHandle
+    | "desktop"
+    | "documents"
+    | "downloads"
+    | "music"
+    | "pictures"
+    | "videos",
+|};
+
+type WindowFilePickerOptions = WindowBaseFilePickerOptions & {|
+  excludeAcceptAllOption?: boolean,
+  types?: Array<WindowFileSystemPickerFileType>,
+|};
+
+type WindowOpenFilePickerOptions = WindowFilePickerOptions & {|
+  multiple?: boolean,
+|};
+
+type WindowSaveFilePickerOptions = WindowFilePickerOptions & {|
+  suggestedName?: string,
+|};
+
+type WindowDirectoryFilePickerOptions = WindowBaseFilePickerOptions & {|
+  mode?: "read" | "readwrite",
+|};
+
+// https://wicg.github.io/file-system-access/#api-showopenfilepicker
+declare function showOpenFilePicker(
+  options?: WindowOpenFilePickerOptions
+): Promise<Array<FileSystemFileHandle>>;
+
+// https://wicg.github.io/file-system-access/#api-showsavefilepicker
+declare function showSaveFilePicker(
+  options?: WindowSaveFilePickerOptions
+): Promise<FileSystemFileHandle>;
+
+// https://wicg.github.io/file-system-access/#api-showdirectorypicker
+declare function showDirectoryPicker(
+  options?: WindowDirectoryFilePickerOptions
+): Promise<FileSystemDirectoryHandle>;     
+
 /* Notification */
 type NotificationPermission = 'default' | 'denied' | 'granted';
 type NotificationDirection = 'auto' | 'ltr' | 'rtl';