`dart format`: comment on return of arrow function

[fishythefish]

In this CL, dart format produces the following:

List<E> _toListGrowable() =>
// slice(0) is slightly faster than slice()
JSArray<E>.markGrowable(JS('', '#.slice(0)', this));

It used to be formatted like this:

List<E> _toListGrowable() =>
    // slice(0) is slightly faster than slice()
    JSArray<E>.markGrowable(JS('', '#.slice(0)', this));

cc @rakudrama

[Reprevise]

Duplicate of https://github.com/dart-lang/dart_style/issues/1645