🔀 address issue #30; test issues #1, #10, #13

[FireJuun]

This PR should help users who are using flutter_web_auth_2 on macOS.

When first running FlutterWebAuth2.authenticate() with version 2.1.4 on macOS, I was getting the exact issue as was noted in issue #30:

flutter_web_auth_2/FlutterWebAuth2Plugin.swift:39: Fatal error: Unexpectedly found nil while unwrapping an Optional value

I believe this platform exception is what you were referencing in issue #10, though there isn't enough detail to know this for sure. Also of note, Chrome is my default browser (testing #13).

To address issue #30, I did the following:

• 🧱 deleted example/macos folder, then ran flutter create .
  • In current state, the macos Podfile was causing a lot of errors
  • The simplest fix here was to allow Flutter to recreate all of its dependencies
• ⬆️ upgraded all macOS platform dependencies to 10.15
  • This requirement was already set by the macos/Classes/FlutterWebAuth2Plugin.swift file

Of note, while making these changes, I also addressed this issue in the test/flutter_web_auth_2_test.dart file:

• 🐛 add null safety to TestDefaultBinaryMessengerBinding

The end result is a working example for macOS in Chrome:

[ThexXTURBOXx]

First of all: Thanks for your PR! However, this does not seem to fix anything in flutter_web_auth_2 directly, but rather addresses some issues in the example and tests.
The issues you mentioned will then still persist in other users' devices. Do you have any idea to fix the underlying issue instead of just the example and tests?