Add FLIPPER_PORTS env var to desktop app

Summary:
Part 3 and final part of adding customizable ports. To use this, the iOS / Android apps have to also be started with the same custom ports.

Example usage: `FLIPPER_PORTS=8189,8188 yarn start`

Reviewed By: passy

Differential Revision: D13801761

fbshipit-source-id: 3dd80a3001ed0855e54cc568fa94eb6fac5fc7f1

src/utils/__tests__/environmentVariables.node.js

@@ -0,0 +1,36 @@
+/**
+ * Copyright 2018-present Facebook.
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ * @format
+ */
+
+import {parseFlipperPorts} from '../environmentVariables';
+
+test('Valid port overrides are parsed correctly', () => {
+  const overrides = parseFlipperPorts('1111,2222');
+  expect(overrides).toEqual({insecure: 1111, secure: 2222});
+});
+
+test('Malformed numbers are ignored', () => {
+  const malformed1 = parseFlipperPorts('1111,22s22');
+  expect(malformed1).toBe(undefined);
+
+  const malformed2 = parseFlipperPorts('11a11,2222');
+  expect(malformed2).toBe(undefined);
+});
+
+test('Wrong number of values is ignored', () => {
+  const overrides = parseFlipperPorts('1111');
+  expect(overrides).toBe(undefined);
+});
+
+test('Empty values are ignored', () => {
+  const overrides = parseFlipperPorts('1111,');
+  expect(overrides).toBe(undefined);
+});
+
+test('Negative values are ignored', () => {
+  const overrides = parseFlipperPorts('-1111,2222');
+  expect(overrides).toBe(undefined);
+});