Better syntax highlighting for Dart (#1959)

Remove unmaintained Dart syntax submodule. Add elMuso/Dartlight as submodule. Update Dart syntax highlighting. Add changes to changelog.

.gitmodules

@@ -67,9 +67,6 @@
 [submodule "assets/syntaxes/JavaScript (Babel)"]
 	path = assets/syntaxes/02_Extra/JavaScript (Babel)
 	url = https://github.com/babel/babel-sublime
-[submodule "assets/syntaxes/Dart"]
-	path = assets/syntaxes/02_Extra/Dart
-	url = https://github.com/guillermooo/dart-sublime-bundle
 [submodule "assets/syntaxes/FSharp"]
 	path = assets/syntaxes/02_Extra/FSharp
 	url = https://github.com/hoest/sublimetext-fsharp
@@ -236,3 +233,6 @@
 [submodule "assets/syntaxes/02_Extra/MediaWiki"]
 	path = assets/syntaxes/02_Extra/MediaWiki
 	url = https://github.com/tosher/Mediawiker.git
+[submodule "assets/syntaxes/02_Extra/Dart"]
+	path = assets/syntaxes/02_Extra/Dart
+	url = https://github.com/elMuso/Dartlight.git

CHANGELOG.md

@@ -36,6 +36,7 @@
 - LiveScript, see #1915 (@Enselic)
 - MediaWiki, see #1925 (@sorairolake)
 - The `requirements.txt` syntax has been removed due to incompatible license requirements.
+- Dart, new highlighter, see #1959 (@Ersikan)
 
 ## New themes