40 columns                              |
>>> Empty body.
extension  A  on  B  {


  }
<<<
extension A on B {}
>>> Members.
extension A on B{z() => 0;f() { body; }}
<<<
extension A on B {
  z() => 0;
  f() {
    body;
  }
}
>>> Insert blank line before and after extension.
var x = 1;
extension A on B {}
var y = 2;
<<<
var x = 1;

extension A on B {}

var y = 2;
>>> Unsplit type parameters.
extension  A  <  T  ,  S  >  on  B  {}
<<<
extension A<T, S> on B {}
>>> Split type parameters.
extension Extension<LongTypeParameter, Another> on BaseClass {}
<<<
extension Extension<
  LongTypeParameter,
  Another
>
    on BaseClass {}
>>> Unnamed.
extension on String {}
<<<
extension on String {}
>>> Unnamed with type parameters.
extension  <  T  ,  S  >  on  B {}
<<<
extension<T, S> on B {}
>>> Split at `on`.
extension SomeExtension on VeryLongClass {}
<<<
extension SomeExtension
    on VeryLongClass {}
>>> Unsplit generic on type.
extension SomeExtension on C<int> {}
<<<
extension SomeExtension on C<int> {}
>>> Split before `on` on generic on type.
extension SomeExtension on C<SomeLongClass> {}
<<<
extension SomeExtension
    on C<SomeLongClass> {}
>>> Split in generic on type.
extension SomeExtension on C<VeryLongType, AnotherLongType> {}
<<<
extension SomeExtension
    on
        C<
          VeryLongType,
          AnotherLongType
        > {}