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/track.dart

@@ -12,7 +12,7 @@ class TrackDimension {
 
 /// Wrapper around a MediaStreamTrack with additional metadata.
 class Track {
-  static const ScreenShareName = "screen";
+  static const screenShareName = 'screen';
 
   String name;
   TrackType kind;
@@ -38,19 +38,17 @@ class Track {
   }
 
   String getCid() {
-    var cid = _cid;
+    var cid = _cid ?? mediaTrack.id;
+
     if (cid == null) {
-      cid = mediaTrack.id;
-    }
-    if (cid == null) {
-      var uuid = Uuid();
+      const uuid = Uuid();
       cid = uuid.v4();
       _cid = cid;
     }
     return cid;
   }
 
-  stop() {
+  void stop() {
     mediaTrack.stop();
   }
 }