Clear future callbacks when the future is completed.

sdk/lib/async/future_impl.dart

Index: sdk/lib/async/future_impl.dart
diff --git a/sdk/lib/async/future_impl.dart b/sdk/lib/async/future_impl.dart
index 5fcbe3dc1b7ffd89fbc9f507b55453fba00d9a7f..a1f09a4fc76aaa20d773a3e7910a02b4e9214a0d 100644
--- a/sdk/lib/async/future_impl.dart
+++ b/sdk/lib/async/future_impl.dart
@@ -128,10 +128,10 @@ class _Future<T> implements Future<T> {
   // If we store the type of a closure in the state field (where there are
   // still bits left), we can just store two closures instead of using 4
   // fields of which 2 are always null.
-  final _FutureOnValue _onValueCallback;
-  final _FutureErrorTest _errorTestCallback;
-  final Function _onErrorCallback;
-  final _FutureAction _whenCompleteActionCallback;
+  _FutureOnValue _onValueCallback;
+  _FutureErrorTest _errorTestCallback;
+  Function _onErrorCallback;
+  _FutureAction _whenCompleteActionCallback;
 
   _FutureOnValue get _onValue => _isChained ? null : _onValueCallback;
   _FutureErrorTest get _errorTest => _isChained ? null : _errorTestCallback;
@@ -578,6 +578,10 @@ class _Future<T> implements Future<T> {
         }
         // If we changed zone, oldZone will not be null.
         if (oldZone != null) Zone._leave(oldZone);
+        listener._onValueCallback = null;
+        listener._errorTestCallback = null;
+        listener._onErrorCallback = null;
+        listener._whenCompleteActionCallback = null;
 
         if (isPropagationAborted) return;
         // If the listener's value is a future we need to chain it. Note that