non_constant_map_element

The elements in a const map literal must be constant.

The analyzer produces this diagnostic when an if element or a spread element in a constant map isn't a constant element.

The following code produces this diagnostic because it's attempting to spread a non-constant map:

var notConst = <int, int>{};
var map = const <int, int>{...notConst};

Similarly, the following code produces this diagnostic because the condition in the if element isn't a constant expression:

bool notConst = true;
var map = const <int, int>{if (notConst) 1 : 2};

If the map needs to be a constant map, then make the elements constants. In the spread example, you might do that by making the collection being spread a constant:

const notConst = <int, int>{};
var map = const <int, int>{...notConst};

If the map doesn't need to be a constant map, then remove the const keyword:

bool notConst = true;
var map = <int, int>{if (notConst) 1 : 2};