Add Factorial in Dart (#4975)

Author: gangaasoonu

archive/d/dart/factorial.dart

@@ -0,0 +1,22 @@
+// Issue 4969
+void main(List<String> args){
+  const String error_message = "Usage: please input a non-negative integer";
+  try{
+
+    int factorial = 1;
+    int num_needed = int.parse(args[0]);
+    if (num_needed.isNegative == true){
+      print(error_message);
+    }
+    else{
+      for (int i_index = 1; i_index <= num_needed; i_index ++){
+        factorial *= i_index;
+      }
+      print(factorial.toString());
+    }
+
+  }
+  catch(e){
+    print(error_message);
+  }
+}