linearSearch in Dart

[cyberianzed]

Description

linear search is a method for finding an element within a list.
Written in Dart.

Checklist:

• My code follows the style guidelines of this project
• I have added my code in its respective language folder.
• I have mentioned a short description about the code at the beginning of the program.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• My changes generate no new warnings.
• I have not deleted any files that already exist in the repository.