Name extensions using UpperCamelCase DRT-W1076

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> {
  // ...
}