create 0028-find-the-index-of-the-first-occurrence-in-a-string.dart

[bansalgaurav852]

• File(s) Modified: 0002-add-two-numbers.dart,0003-longest-substring-without-repeating-characters.dart,0028-find-the-index-of-the-first-occurrence-in-a-string.dart,0004-median-of-two-sorted-arrays.dart
• Language(s) Used: dart
• Submission URL: https://leetcode.com/problems/add-two-numbers/solutions/5557791/dart-solution,https://leetcode.com/problems/longest-substring-without-repeating-characters/solutions/5557762/dart-solution,https://leetcode.com/problems/find-the-index-of-the-first-occurrence-in-a-string/solutions/5557800/dart-solution,https://leetcode.com/problems/median-of-two-sorted-arrays/solutions/5557815/dart-solution

Important

Please make sure the file name is lowercase and a duplicate file does not already exist before merging.