Frog permits local function to be overwritten

[rakudrama]

Local functions should be final.

main() {
  f() => 1;
  f = null;
  print('f = $f');
}

$ frogsh bug1a.dart
f = null
$ dart_bin bug1a.dart
'bug1a.dart': Error: line 3 pos 5: Left hand side of '=' is not assignable
  f = null;
    ^

$

[jmesserly]

I don't think we check 'final' for anything yet :)

[jmesserly]

Added Fixed label.