More tests and doc for futures (followup to http://codereview.chromium.org/10517006/)

corelib/src/future.dart

Index: corelib/src/future.dart
===================================================================
--- corelib/src/future.dart	(revision 8412)
+++ corelib/src/future.dart	(working copy)
@@ -19,7 +19,7 @@
  * (producing an exception, which may be handled with [handleException]).
  * Callbacks passed to [onComplete] will be invoked in either case.
  *
- * When a future completes:
+ * When a future completes, the following actions happen in order:
  *
  *   1. if the future suceeded, handlers registered with [then] are called.
  *   2. if the future failed, handlers registered with [handleException] are
@@ -69,7 +69,7 @@
    * If this future is complete and has a value, then [onValue] is called
    * with the value.
    */
-  void then(void onValue(T value));
+  void then(void onSuccess(T value));
 
   /**
    * If this future is complete and has an exception, then call [onException].