futures: add more types to chain/transform, some additional cleanup.

corelib/src/implementation/future_implementation.dart

 // Copyright 2011 Google Inc. All Rights Reserved.
 // Dart core library.
 
-/**
- * Thrown if client tries to obtain value or exception
- * before a future has completed.
- */
-class FutureNotCompleteException implements Exception {
-  FutureNotCompleteException() {}
-  String toString() => "Exception: future has not been completed";
-}
-
-/**
- * Thrown if a completer tries to set the value on
- * a future that is already complete.
- */
-class FutureAlreadyCompleteException implements Exception {
-  FutureAlreadyCompleteException() {}
-  String toString() => "Exception: future already completed";
-}
-
-
 class FutureImpl<T> implements Future<T> {
 
   bool _isComplete;
 
   /**
    * Value that was provided to this Future by the Completer
    */
   T _value;
 
   /**
@@ skipping 160 matching lines @@
   }
 
   void complete(T value) {
     _futureImpl._setValue(value);
   }
 
   void completeException(var exception) {
     _futureImpl._setException(exception);
   }
 }