Actually you can use if/else and switch and any other statement inline in dart / flutter.
Use a builder

  Widget build(BuildContext context) {
    return Container(
      child: Builder(
        builder: (context) {
          if (true) //
            return "tis true";

          return "anything but true";
        },
      ),
    );
  }

or an immediate anonymous function

  Widget build(BuildContext context) {
    return Container(child: () {
      if (value == 1) {
        return Text('tis true');
      }
      return Text('anything but true');
    }());
  }

I would heavily recommend against putting too much logic directly with your UI 'markup' but I found that type inference in Dart needs a little bit of work so it can be sometimes useful in scenarios like that.
Use the ternary operator

condition ? Text("True") : null,

Use If or For statements or spread operators in collections

children: [
  ...manyItems,
  oneItem,
  if(canIKickIt)
    ...kickTheCan
  for (item in items)
    Text(item)

Use a method

child: getWidget()

Widget getWidget() {
  if (x > 5) ...
  //more logic here and return a Widget

EDIT - new with Dart 3.0

Dart 3.0 adds new functionality such as if-case & switch expressions in order to use pattern matching, destructuring, guard clauses.
https://dart.dev/language/patterns
Switch Expressions

We can now use switch as an expression. https://dart.dev/language/branches#switch-expressions

child: switch (task) {
  Task_completed(:date) => Text("completed on $date"),
  Task_overdue(:priority) => Text("$priority"),
},

if-case (new to Dart 3.0)

Use if-case instead of switch when you have a single or small number of conditions to check. https://dart.dev/language/branches#if-case

return ListView(children: [
  Text('header'),
  if (pair case [int x, int y]) //
    Text('Coordinates: ($x, $y)') //
  else
    Text('No coordinates given'),
]);