What is the Dart null checking idiom or best practice?

Question

I have the following form of assignment & null checks to avoid double lookups in my maps.
Is there a better or more idiomatic way to do this in Dart?

Answer 1

There are now 4 null aware operators

??     provides a default if the subject is null

return subject ?? "defaultIfNull";

??=     sets the subject to a default only if the subject is null

This is similar to ?? but sets the subject variable to a default if it is null.

subject ??= "defaultIfNull";

?.     avoid an exception if subject is null when accessing subject's property

object?.x will return null if object is null, object.x would cause an exception if object were null

...?     from a spread collection, avoid a null item in the final list if the subject list is null

the result of the following

[
  ...[1, 2],
  null,
]

is [1, 2, null]

to avoid the null value use ...?

var resultingList = [
  ...[1, 2],
  ...?subjectList,
];