controlflow.dart

## Program 3: Determine Grade Based on Marks
Write a Dart program to determine the grade based on a student's marks. The program should print out the appropriate grade according to the following criteria:
- If the marks are greater than 85, print "Excellent".
- If the marks are between 75 and 85 (inclusive), print "Very Good".
- If the marks are between 65 and 75 (inclusive), print "Good".
- If the marks are below 65, print "Average".