Remove the ability to use the dart formatter to format a subset of a file

[jwren]

While a user request and a nice feature, when it doesn't work, the UX isn't great. See dart-lang/dart_style#1742

@pq @munificent

[jwren]

Task here is to revert (but not in an automated manner) this change: JetBrains/intellij-plugins@59ecda7