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/options.dart

@@ -1,6 +1,6 @@
 /// Options when creating a LocalVideoTrack.
 class LocalVideoTrackOptions {
-  CameraPosition position = CameraPosition.FRONT;
+  CameraPosition position = CameraPosition.front;
   VideoParameter params;
 
   LocalVideoTrackOptions({
@@ -16,16 +16,16 @@ class LocalVideoTrackOptions {
   }
 
   Map<String, dynamic> get mediaConstraints {
-    return {
-      "mandatory": params.mediaConstraints,
-      "facingMode": position == CameraPosition.FRONT ? "user" : "environment",
+    return <String, dynamic>{
+      'mandatory': params.mediaConstraints,
+      'facingMode': position == CameraPosition.front ? 'user' : 'environment',
     };
   }
 }
 
 enum CameraPosition {
-  FRONT,
-  BACK,
+  front,
+  back,
 }
 
 class VideoParameter {
@@ -38,14 +38,14 @@ class VideoParameter {
     this.width,
     this.height,
     this.fps, {
-    int? bitrate,
-  }) : this.bitrate = bitrate;
+    this.bitrate,
+  });
 
   Map<String, dynamic> get mediaConstraints {
-    return {
-      "minWidth": this.width,
-      "minHeight": this.height,
-      "minFrameRate": this.fps,
+    return <String, dynamic>{
+      'minWidth': width,
+      'minHeight': height,
+      'minFrameRate': fps,
     };
   }
 }