The Flexible does the trick
new Container(
child: Row(
children: <Widget>[
Flexible(
child: new Text("A looooooooooooooooooong text"))
],
));
This is the official doc https://flutter.dev/docs/development/ui/layout#lay-out-multiple-widgets-vertically-and-horizontally on how to arrange widgets.
Remember that Flexible and also Expanded, should only be used within a Column, Row or Flex, because of the Incorrect use of ParentDataWidget.
The solution is not the mere Flexible