rest_element_in_map_pattern

A map pattern can't contain a rest pattern.

The analyzer produces this diagnostic when a map pattern contains a rest pattern. Map patterns match a map with more keys than those explicitly given in the pattern (as long as the given keys match), so a rest pattern is unnecessary.

The following code produces this diagnostic because the map pattern contains a rest pattern:

void f(Map<int, String> x) {
  if (x case {0: _, ...}) {}
}

Remove the rest pattern:

void f(Map<int, String> x) {
  if (x case {0: _}) {}
}