Fix FS in browser

packages/core/fs/package.json

@@ -40,5 +40,8 @@
   },
   "peerDependencies": {
     "@parcel/core": "^2.0.0"
+  },
+  "browser": {
+    "./src/NodeFS.js": "./src/NodeFS.browser.js"
   }
 }

packages/core/fs/src/MemoryFS.js

@@ -1,4 +1,5 @@
 // @flow
+/* global MessageChannel:readonly */
 
 import type {FileSystem, FileOptions, ReaddirOptions, Encoding} from './types';
 import type {FilePath} from '@parcel/types';
@@ -17,6 +18,27 @@ import nullthrows from 'nullthrows';
 import EventEmitter from 'events';
 import {findAncestorFile, findNodeModule, findFirstFile} from './find';
 
+let SharedBuffer: Class<ArrayBuffer> | Class<SharedArrayBuffer>;
+// $FlowFixMe[prop-missing]
+if (process.browser) {
+  SharedBuffer = ArrayBuffer;
+  // Safari has removed the constructor
+  if (typeof SharedArrayBuffer !== 'undefined') {
+    let channel = new MessageChannel();
+    try {
+      // Firefox might throw when sending the Buffer over a MessagePort
+      channel.port1.postMessage(new SharedArrayBuffer(0));
+      SharedBuffer = SharedArrayBuffer;
+    } catch (_) {
+      // NOOP
+    }
+    channel.port1.close();
+    channel.port2.close();
+  }
+} else {
+  SharedBuffer = SharedArrayBuffer;
+}
+
 const instances: Map<number, MemoryFS> = new Map();
 let id = 0;
 
@@ -904,15 +926,12 @@ class Directory extends Entry {
 }
 
 function makeShared(contents: Buffer | string): Buffer {
-  if (
-    typeof contents !== 'string' &&
-    contents.buffer instanceof SharedArrayBuffer
-  ) {
+  if (typeof contents !== 'string' && contents.buffer instanceof SharedBuffer) {
     return contents;
   }
 
   let length = Buffer.byteLength(contents);
-  let shared = new SharedArrayBuffer(length);
+  let shared = new SharedBuffer(length);
   let buffer = Buffer.from(shared);
   if (typeof contents === 'string') {
     buffer.write(contents);

packages/core/fs/src/NodeFS.browser.js

@@ -0,0 +1,9 @@
+// @flow
+import type {FileSystem} from './types';
+
+// $FlowFixMe[prop-missing] handled by the throwing constructor
+export class NodeFS implements FileSystem {
+  constructor() {
+    throw new Error("NodeFS isn't available in the browser");
+  }
+}