The if and spread elements weren't supported in constant expressions until version 2.5.0, but this code is required to be able to run on earlier versionsDRT-W1618
The analyzer produces this diagnostic when an if or spread element inside a constant context is found in code that has an SDK constraint whose lower bound is less than 2.5.0. Using an if or spread element inside a constant context wasn't supported in earlier versions, so this code won't be able to run against earlier versions of the SDK.
Example
Here's an example of a pubspec that defines an SDK constraint with a lower bound of less than 2.5.0:
environment:
sdk: '>=2.4.0 <2.6.0'
In the package that has that pubspec, code like the following produces this diagnostic:
const a = [1, 2];
const b = [...a];
Common fixes
If you don't need to support older versions of the SDK, then you can increase the SDK constraint to allow the syntax to be used:
environment:
sdk: '>=2.5.0 <2.6.0'
If you need to support older versions of the SDK, then rewrite the code to not make use of those elements:
const a = [1, 2];
const b = [1, 2];
If that isn't possible, change the code so that the element isn't in a constant context:
const a = [1, 2];
var b = [...a];