Apply Flutter best practices [Non-Breaking] (#2)

* Add flutter_lints package

* Use key in widget constructors

lint: use_key_in_widget_constructors

* Unnecessary new keyword

lint: unnecessary_new

* Prefer const with constant constructors

* Exclude protobuf files from analyzer

* Private field could be final

* Annotate overridden members

* Use initializing formals when possible

* Don't access members with `this` unless avoiding shadowing

* Prefer is! operator

* Use isEmpty instead of length

* Use collection literals when possible

* Revert comment changes

* Cleaner null testing

* Misc fixes

* Avoid using `forEach` with a function literal

* Prefer `final` over `var` where applicable

* Enforce stricter type-checking

* Ignore VS Code files

* Flutter format

* Prefer using lowerCamelCase for constant names

* flutter format -l 100

* Disable `avoid_print` for examples

* Slight improvements to example

Should solve lint error: no_logic_in_create_state

lib/src/track/local_audio_track.dart

@@ -9,19 +9,18 @@ class LocalAudioTrack extends AudioTrack {
       : super(name, track, stream);
 
   /// Creates a new audio track from the default audio input device.
-  static Future<LocalAudioTrack> createTrack(
-      [LocalAudioTrackOptions? options]) async {
+  static Future<LocalAudioTrack> createTrack([LocalAudioTrackOptions? options]) async {
     try {
-      var stream = await navigator.mediaDevices.getUserMedia({
-        "audio": true,
-        "video": false,
+      final stream = await navigator.mediaDevices.getUserMedia(<String, dynamic>{
+        'audio': true,
+        'video': false,
       });
 
-      if (stream.getAudioTracks().length == 0) {
+      if (stream.getAudioTracks().isEmpty) {
         return Future.error(TrackCreateError());
       }
 
-      return LocalAudioTrack("", stream.getAudioTracks().first, stream);
+      return LocalAudioTrack('', stream.getAudioTracks().first, stream);
     } catch (e) {
       return Future.error(e);
     }