camel_case_extensions  
            
            Name extensions using UpperCamelCase.
This rule is available as of Dart 2.6.0.
Rule sets: core, recommended, flutter
Details
From Effective Dart:
DO name extensions using UpperCamelCase.
Extensions should capitalize the first letter of each word (including the first word), and use no separators.
GOOD:
extension MyFancyList<T> on List<T> { 
  // ... 
}
extension SmartIterable<T> on Iterable<T> {
  // ...
}
Usage
To enable the camel_case_extensions rule,
add camel_case_extensions under linter > rules in your
analysis_options.yaml
file:
linter:
  rules:
    - camel_case_extensions