Specs: Implement the Dispatcher classes.

sky/specs/script.md

 Sky Script Language
 ===================
 
 The Sky script language is Dart.
 
 The way that Sky integrates the module system with its script language
 is described in [modules.md](modules.md).
 
 When an method defined as ``external`` receives an argument, it must
 type-check it, and, if the argument's value is the wrong type, then it
@@ skipping 87 matching lines @@
   to a superclass' constructor, wherein the subclass' constructor has
   the same arguments as the superclass' constructor and does nothing
   but invoke that superclass' constructor with the same arguments. The
   syntax for defining this is, within the class body for a class
   called ClassName:
 
 ```dart
      ClassName = SuperclassName;
      ClassName.namedConstructor = SuperclassName.otherNamedConstructor;
 ```
-
-* It is assumed that the standard library includes something that
-  matches this pattern:
-
-```dart
-class DispatcherController<T> {
-  Dispatcher<T> _dispatcher;
-  Dispatcher<T> get dispatcher => _dispatcher;
-
-  void add(T data) {
-    // ...
-  }
-}
-typedef bool Filter<T>(T t);
-typedef void Handler<T>(T t);
-class Dispatcher<T> {
-  Dispatcher<T> where(Filter<T> filter) { /*...*/ return this; }
-  void listen(Handler<T> handler) { /* ... */ }
-}
-class ExceptionListException<T> extends Exception with IterableMixin<T> {
-  List<T> _exceptions;
-  void add(T exception) {
-    if (_exceptions == null)
-      _exceptions = new List<T>();
-    _exceptions.add(exception);
-  }
-  int get length => _exceptions == null ? 0 : _exceptions.length;
-  Iterator<T> iterator() => _exceptions.iterator();
-}
-```