feat(transpiler): map type declarations to dart.

ES6's const is Dart's final, so the type declarations are changed to
reflect that:

  untyped                   typed
var a -> var a;           var a:T -> T a;
let a -> var a;           let a:T -> T a;
const a -> final a;       const a:T -> final T a;

Still needed is support for Dart's const. Temporary moved all const
expressions to the .dart/.es6 specific files.
Should this be done in a separate transformer rather than in the writer ?