rename RetrieveFlags.option to RetrieveFlags.optionWithDefault

[devoncarew]

• rename RetrieveFlags.option to RetrieveFlags.optionWithDefault

This rename will let us land dart-archive/args#248 in the sdk repo; without this, the new option() method is used instead of the RetrieveFlags.option extension method, and the two have different return types (String? vs String).

output: ../../third_party/pkg/pub/lib/src/command/lish.dart:48:57: Error: The argument type 'String?' can't be assigned to the parameter type 'String' because 'String?' is nullable and 'String' isn't.
        return validateAndNormalizeHostedUrl(argResults.option('server'));
                                                        ^

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[sigurdm]

Rolling into sdk here: https://dart-review.googlesource.com/c/sdk/+/357326

[devoncarew]

Rolling into sdk here: https://dart-review.googlesource.com/c/sdk/+/357326

Thanks!