Named constructors are incorrectly transpiled

[JeroMiya]

Named constructors in dart are actual constructors. While we have to transpile them into static methods in C#, we also have to generate the corresponding constructor. Otherwise, we'll get syntax errors if there is no default constructor or if fields are readonly fields or properties.

The named constructor in Dart should be transpiled into a regular private constructor in C#, followed by a static method representing the named constructor which forwards the parameters to the internal constructor.