[swift2objc] Support unlabeled parameters

[liamappelbe]

Swift function params have both an internal name (used inside the function), and an external label (used like a Dart named parameter when invoking the function). Either one, but not both, can be omitted. If the internal name is omitted, then the external label is also the internal name. If the external label is omitted (or rather, set to _), then the parameter is invoked like a Dart positional param.

public foo(label1 name1: Int, label2: Int, _ name3: Int) {
  // In here we have name1, label2, and name3.
}

foo(label1: 123, label2: 456, 789);

• On the parsing side, we supported these cases for initializers but not functions/methods. I've unified the parsing logic by renaming/moving parseInitializerParams to parseFunctionParams.
  • This function is unchanged except that function declaration fragments can end with -> ReturnType. So the check at the end of parsing has to allow that case.
• On the codegen side, the initializer/function/method signatures were generating correctly, but the way we were invoking the wrapped function was incorrect
  • I factored out the invocation stuff into generateInvocationParams, and fixed the bug by checking if the label is _.

[coveralls]

coverage: 88.828% (-0.01%) from 88.84%
when pulling c44a575 on anonparam
into 071dcf3 on main.