Request: Allow constructors with parameters

[seanmccleary]

The requirement that classes have a default constructor with no arguments makes it difficult to enforce certain invariants in those classes.

For example, if it's a guarantee in your application that your "User" class will always have an attribute called "emailAddress" which is set, then it shouldn't be possible to construct a User without the email address. The constructor should look something like this:

class User {
  String emailAddress;
  User(this.emailAddress);
}

Yet in order to make use of this rpc library, the class must have a parameterless constructor, which allows one to incorrectly construct a User.