Handle defered handling of exceptions (where the exception is thrown before the

corelib/src/implementation/future_implementation.dart

Index: corelib/src/implementation/future_implementation.dart
diff --git a/corelib/src/implementation/future_implementation.dart b/corelib/src/implementation/future_implementation.dart
index 47c1e5d8c07ac628f806556c1f7e1b1e0de3fc1c..d4b7ad0c958dc0ff9ee70ee6da7574743965e4a7 100644
--- a/corelib/src/implementation/future_implementation.dart
+++ b/corelib/src/implementation/future_implementation.dart
@@ -85,13 +85,15 @@ class FutureImpl<T> implements Future<T> {
       onComplete(value);
     } else if (!isComplete) {
       _listeners.add(onComplete);
-    } else if (!_exceptionHandled) {

[Siggi Cherem (dart-lang), 2012/01/10 20:39:27]

I believe we don't want to do this part of the change.

Basically we are worried about users of [Future] that don't register any
exception handlers at first (it wasn't initially part of their design), but
later end up inadvertently ignoring exceptions. 

It's unfortunate that this requires an ordering (add exception handlers before
calling 'then'), but I still believe exposing exceptions out-weights this.

-      throw _exception;
     }
   }
 
   void handleException(void onException(Object exception)) {
-    _exceptionHandlers.add(onException);
+    if (null !== _exception) {

[Siggi Cherem (dart-lang), 2012/01/10 20:39:27]

(nit): keeping the style consistent with the rest of the dart code, swap the 2
expressions (_expression !== null)

+      if (!_exceptionHandled) _exceptionHandled = onException(exception);
+    } else {
+      _exceptionHandlers.add(onException);
+    }
   }
 
   void _complete() {