Set featureFlag default to null so that it can be resolved by settings (#1838)

* set featureFlag default to null so that it can be resolved by settings

* added tests

Author: TylerLeonhardt

package.json

@@ -738,7 +738,7 @@
         },
         "powershell.developer.featureFlags": {
           "type": "array",
-          "default": [],
+          "default": null,
           "description": "An array of strings that enable experimental features in the PowerShell extension."
         },
         "powershell.developer.powerShellExeIsWindowsDevBuild": {

test/settings.test.ts

@@ -0,0 +1,22 @@
+/*---------------------------------------------------------
+ * Copyright (C) Microsoft Corporation. All rights reserved.
+ *--------------------------------------------------------*/
+
+import * as assert from "assert";
+import Settings = require("../src/settings");
+
+suite("Settings module", () => {
+    test("Settings load without error", () => {
+        assert.doesNotThrow(Settings.load);
+    });
+
+    // TODO: Remove this test when PSReadLine is in stable
+    test("PSReadLine featureFlag set correctly", () => {
+        const settings: Settings.ISettings = Settings.load();
+        if (process.platform === "win32") {
+            assert.deepEqual(settings.developer.featureFlags, ["PSReadLine"]);
+        } else {
+            assert.deepEqual(settings.developer.featureFlags, []);
+        }
+    });
+});