const_spread_expected_map

A map is expected in this spread.

The analyzer produces this diagnostic when the expression of a spread operator in a constant map evaluates to something other than a map.

The following code produces this diagnostic because the value of map1 is null, which isn't a map:

const dynamic map1 = 42;
const Map<String, int> map2 = {...map1};

Change the expression to something that evaluates to a constant map:

const dynamic map1 = {'answer': 42};
const Map<String, int> map2 = {...map1};