Syntax for optional parameters and required named parameters is verbose and unfamiliar

The current syntax for optional positional parameters is:

```dart
foo([bool optional = false])
```

The current syntax for required named parameters is:

```dart
foo({@required bool optional = false})
```

Both syntaxes are not used in any other mainstream language that I know of and are unnecessary verbose compared to possible alternatives.

Review the issue's examples of optional positional and required named parameter syntax and the discussion that follows them. Determine whether a concrete alternative has been agreed on; done would require a settled language-design proposal, but no files, tests, or implementation entry points are identified here.

Written by the indexing model from the issue text.