fileStorage/reducers: drop use of Set

Using set causes problems with tooling that mocks the global app state,
so stick with arrays.

src/editor/sagas.test.ts

@@ -140,7 +140,7 @@ describe('setEditSession', () => {
     it('should wait for storage to be initialized', async () => {
         const mockEditor = mock<monaco.editor.ICodeEditor>();
         const saga = new AsyncSaga(editor, {
-            fileStorage: { isInitialized: false, fileNames: new Set() },
+            fileStorage: { isInitialized: false, fileNames: [] },
         });
 
         saga.put(setEditSession(mockEditor));
@@ -155,7 +155,7 @@ describe('setEditSession', () => {
     it('should load main.py', async () => {
         const mockEditor = mock<monaco.editor.ICodeEditor>();
         const saga = new AsyncSaga(editor, {
-            fileStorage: { isInitialized: true, fileNames: new Set(['main.py']) },
+            fileStorage: { isInitialized: true, fileNames: ['main.py'] },
         });
 
         saga.put(setEditSession(mockEditor));
@@ -175,7 +175,7 @@ describe('setEditSession', () => {
     it('should not raise error if main.py does not exist', async () => {
         const mockEditor = mock<monaco.editor.ICodeEditor>();
         const saga = new AsyncSaga(editor, {
-            fileStorage: { isInitialized: true, fileNames: new Set() },
+            fileStorage: { isInitialized: true, fileNames: [] },
         });
 
         saga.put(setEditSession(mockEditor));

src/editor/sagas.ts

@@ -115,7 +115,7 @@ function* handleSetEditSession(action: ReturnType<typeof setEditSession>): Gener
 
     const fileList = yield* select((s: RootState) => s.fileStorage.fileNames);
 
-    if (!fileList.has(currentFileName)) {
+    if (!fileList.includes(currentFileName)) {
         // The file doesn't exist in storage, so don't try to open it.
         yield* put(didSetEditSession(action.editSession));
         return;