prefer_expression_function_bodies

Unnecessary use of a block function body.

The analyzer produces this diagnostic when the body of a function consists of a single return statement with an expression.

The following code produces this diagnostic because the body of f has a single return statement:

int f() {
  return 0;
}

If the body is complete, then replace the body with an expression body:

int f() => 0;

If the body isn't complete, then add the missing statements.