Improve Ctrl+Shift+Enter

`Ctrl+Shift+Enter` is supposed to complete the current sentence and go to the next line. Currently for Dart it's not working very well, it doesn't add a `;` at the end of the line before jumping to the next line, please improve it if possible.

Start by reproducing Ctrl+Shift+Enter in a Dart file and tracing the IntelliJ plugin's handling of that shortcut. Done means the current Dart sentence receives a trailing semicolon before the editor moves to the next line, without regressing the existing behavior.

Written by the indexing model from the issue text.