Add support for library classes

[theEvilReaper]

Dart allows the creation of library classes. A library class can help to minimize tight coupling and make maintainable code.
In terms of library classes, the part of directive is important. These allow other classes etc. to be included in the library without them being in the library file.

An example of a library may look like this:

Library class:

library test;

import 'dart:core' as core;
import '../test.dart';

Small class

part of test;

class Test {
  bool notZero(int value) {
    return value != 0;
  }
}

Small side node: There is no good explaination from the Dart wiki itself about library classes