not_map_spread

Spread elements in map literals must implement 'Map'.

The analyzer produces this diagnostic when the static type of the expression of a spread element that appears in a map literal doesn't implement the type Map.

The following code produces this diagnostic because l isn't a Map:

var l =  <String>['a', 'b'];
var m = <int, String>{...l};

The most common fix is to replace the expression with one that produces a map:

var l =  <String>['a', 'b'];
var m = <int, String>{...l.asMap()};