Enhanced Default Constructors

I propose to have default constructors
* automatically have named initialzing formal parameters for instance variables declared in the class (at least if they don't have initializer expressions).
* automatically forward parameters to accessible superclass generative constructors (when possible and not shadowed an the initializing formal with the same name introduced above).

This is a solution to #314 (allows you to declare classes with fields without having to write a constructor) and to #469 (forwards superclass constructor parameters). It might even be a solution for *some* parts of #367, but only in the simple cases where you don't need to do anything except storing the non-forwarded parameters.

([Initial design document](https://github.com/dart-lang/language/blob/master/working/0698%20-%20Enhanced%20Default%20Constructors/proposal.md)).