add dart null aware operator

dart.html.markdown

@@ -659,6 +659,11 @@ example33() {
     findVolume33(10,20);//valid
 }
 
+/// Dart has also added feature such as Null aware operators
+var isBool = true;
+var hasString = isBool ?? "default String";
+var hasValue ??= "default Value";
+
 /// Programs have only one entry point in the main function.
 /// Nothing is expected to be executed on the outer scope before a program
 /// starts running with what's in its main function.