TypeReference for a class shouldn't add a ? when calling constructor or class property.

When building code for TypeReference.newInstance/Named and the type had isNullable = true the resulting code becomes Foo?.namedConstructor() which for my use case was not what I wanted.

Here is my use case:

// useNullSafetySyntax = true

final class PluginField extends BaseField {
  PluginField.fromJson(Map<String, dynamic> data, String name)
      : isRequired = data["required"],
        super.fromJson(data, name);
  final bool isRequired;

  @override
  late final TypeReference type = TypeReference((t) {
    t.symbol = "$Plugin";
    t.isNullable = !isRequired;
  });

  @override
  Expression buildInitializer(CodeExpression valueExpression) {
    final expression = type.newInstanceNamed("fromJson", [valueExpression]);
    if (isRequired) return expression;
    return valueExpression.isNotA(refer("$Map")).conditional(literalNull, expression);
  }
}

class Plugin {
  Plugin.fromJson(Map<String, dynamic> data) : name = data["name"];
  final String name;
}

type in this case is shared between a caller building a declaration off of it and buildInitializer building initializer code in a constructor.

The resulting code before this PR for buildInitializer was Plugin?.fromJson(data["value"]) when isRequired = false (isNullable = true).

I think it makes sense to have a TypeReference not add a ? onto the type when calling a constructor or static property but to add it when being used to declare a property.

[x] I’ve reviewed the contributor guide and applied the relevant portions to this PR.

Contribution guidelines:

- See our [contributor guide](https://github.com/dart-lang/.github/blob/main/CONTRIBUTING.md) for general expectations for PRs. - Larger or significant changes should be discussed in an issue before creating a PR. - Contributions to our repos should follow the [Dart style guide](https://dart.dev/guides/language/effective-dart) and use `dart format`. - Most changes should add an entry to the changelog and may need to [rev the pubspec package version](https://github.com/dart-lang/sdk/wiki/External-Package-Maintenance#making-a-change). - Changes to packages require [corresponding tests](https://github.com/dart-lang/.github/blob/main/CONTRIBUTING.md#Testing). Note that many Dart repos have a weekly cadence for reviewing PRs - please allow for some latency before initial review feedback.

dart-lang / code_builder

TypeReference for a class shouldn't add a ? when calling constructor or class property. #458