[analyzer] No error if augmentation adds instance/static methods conflict

[sgrekhov]

The following code produces no errors in the analyzer

import augment 'aug_lib.dart';

class C {
  static int foo() => 42;
}

mixin M {
  static int get foo => 42;
}

main() {
  print(C);
  print(M);
}
// aug_lib.dart

augment library 'main_lib.dart';

augment class C {
  int foo() => 42; // Must be an error, there is static method 'foo'
}

augment mixin M {
  int foo() => 42; // Muxt be an error, there is static method 'foo'
}

[lrhn]

(That's awesome, let's just allow it! dart-lang/language#1711 FTW.)

Alas, until we get that feature, it should be an error.