undefined_extension_setter

The setter '{0}' isn't defined for the extension '{1}'.

The analyzer produces this diagnostic when a static setter is referenced, but isn't defined by the specified extension.

The following code produces this diagnostic because the extension E doesn't declare a static setter named a:

extension E on String {}
​
void f() {
  E.a = 3;
}

If the name of the setter is incorrect, then change it to the name of an existing static setter:

extension E on String {
  static set b(int _) {}
}
​
void f() {
  E.b = 3;
}

If the setter isn't defined, then define it:

extension E on String {
  static set a(int _) {}
}
​
void f() {
  E.a = 3;
}