prefer_function_declarations_over_variables

Use a function declaration rather than a variable assignment to bind a function to a name.

The analyzer produces this diagnostic when a closure is assigned to a local variable and the local variable is not re-assigned anywhere.

The following code produces this diagnostic because the local variable f is initialized to be a closure and isn't assigned any other value:

void g() {
  var f = (int i) => i * 2;
  f(1);
}

Replace the local variable with a local function:

void g() {
  int f(int i) => i * 2;
  f(1);
}