I have looked at these error messages:

foo.dart:1:1: expected identifier, but got 'void'
void x;
^^^^
foo.dart:1:5: Error: Type "void" is only allowed in a return type.
foo(void x) {}
    ^^^^

It is changed to:

foo.dart:1:1: Error: Type 'void' can't be used here because it isn't a return type.
Try removing 'void' keyword or replace it with 'var', 'final', or a type.
void x;
^^^^
foo.dart:1:5: Error: Type 'void' can't be used here because it isn't a return type.
Try removing 'void' keyword or replace it with 'var', 'final', or a type.
foo(void x) {}
    ^^^^

Let me know what you think.