allow optional parameters for types using fromJson()

[johnpryan]

I'm not sure this should really be allowed or should show a warning, but there are cases where fromJson constructors may implement optional parameters:

for example:

class Foo {
  String name;
  int value;
  Foo.fromJson(json, {initValue: false}) {
    name = json['name'];
    value = json['value'];
    if (initValue) {
      value = value ?? 0;
    }
  }
}

@JsonSerializable()
class Person extends Object with _$PersonSerializerMixin {
  final Foo foo;

  Person(this.foo);

  factory Person.fromJson(Map<String, dynamic> json) => _$PersonFromJson(json);
}

class Foo {
  String name;
  int value;
  Foo.fromJson(json, {initValue: false}) {
    name = json['name'];
    value = json['value'];
    if (initValue) {
      value = value ?? 0;
    }
  }
}

currently throws an exception because Foo.fromJson does not have a single parameter.

[kevmoo]

Totally reasonable. Requests

1. Rebase on master
2. Add at least one test to sanity check

[johnpryan]

Thanks for the quick response

1. done
2. done

[kevmoo]

Remove this guy!

[johnpryan]

oh man... fixed.

I decided to squash the changes to hide my shame...

[kevmoo]

will wait for travis to go green before I merge this.

Thanks!