futures: handle exceptions immediately if the future already completed.

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 48c7d405b7c0d58f31143d1268c6ac93c1d66a79..be56fe6f83efc572f8300015bcfe0c4397b5ab34 100644
--- a/corelib/src/implementation/future_implementation.dart
+++ b/corelib/src/implementation/future_implementation.dart
@@ -94,8 +94,15 @@ class FutureImpl<T> implements Future<T> {
     }
   }
 
-  void handleException(void onException(Object exception)) {
-    _exceptionHandlers.add(onException);
+  void handleException(bool onException(Object exception)) {
+    if (_exceptionHandled) return;
+    if (_isComplete) {
+       if (_exception != null) {
+         _exceptionHandled = onException(_exception);
+       }
+    } else {
+      _exceptionHandlers.add(onException);
+    }
   }
 
   void _complete() {