How to conditionally add widgets to a list?

EDIT:

Since Dart 2.2, new syntaxes supports this natively:

Column(
  children: [
    if (foo != null) Text(foo),
    Bar(),
  ],
);

This problem is currently debated on github here.

But for now, you can use dart sync* functions:

Row(
  children: toList(() sync* {
    if (foo == 42) {
      yield Text("foo");
    }
  }),
);

where toList is:

typedef Iterable<T> IterableCallback<T>();

List<T> toList<T>(IterableCallback<T> cb) {
  return List.unmodifiable(cb());
}

Not only it solves the conditional addition problem; it also allows for a "spread operator" thanks to yield*. Example:

List<Widget> foo;

Row(
  children: toList(() sync* {
    yield Text("Hello World");
    yield* foo;
  }),
);

The new Dart syntax allows 'if' in lists, which leads to this simple solution:

Row(
  children: <Widget>[
    if (foo == 42) Text("foo"),
  ],
);

Here's a simpler version I use:

Row(
  children: [
    Text("always included"),
    skipNulls([
      icon,
      label,
    ]),
  ],
);

skipNulls<T>(List<T> items) {
  return items..removeWhere((item) => item == null);
}