Not specifying a type on a method override should not trigger an error

[jacob314]

Instead we should automatically use the type from the base class.

Repro

class LocaleDataException implements Exception {
  final String message;
  LocaleDataException(this.message);
   /* severe: InvalidMethodOverride */ toString() => "LocaleDataException: $message";
}

[vsmenon]

This will change the observable semantics. We could alternatively do a quick fix here instead since we have the info.

[sigmundch]

(btw, this today is reported as an inferrable override)

[alan-knight]

If you're going to force me to put an explicit return type on every toString() method, even if you add it for me, you're going to get a lot of complaining.

[vsmenon]

We now infer return types. Parameter types are left - closing this - please refer to #105