Make Invoker.removeAllOutstandingCallbacks public.

lib/src/backend/invoker.dart

Index: lib/src/backend/invoker.dart
diff --git a/lib/src/backend/invoker.dart b/lib/src/backend/invoker.dart
index 221c52a06a3667638c1ec85ea2efda388906d061..fbf7f20176cc825c2476944b90d52ef9962f2a0f 100644
--- a/lib/src/backend/invoker.dart
+++ b/lib/src/backend/invoker.dart
@@ -120,6 +120,13 @@ class Invoker {
   void removeOutstandingCallback() =>
       _outstandingCallbacks.removeOutstandingCallback();
 
+  /// Removes all outstanding callbacks, for example when an error occurs.
+  ///
+  /// Future calls to [addOutstandingCallback] and [removeOutstandingCallback]
+  /// will be ignored.
+  void removeAllOutstandingCallbacks() =>
+      _outstandingCallbacks.removeAllOutstandingCallbacks();
+
   /// Runs [fn] and returns once all (registered) outstanding callbacks it
   /// transitively invokes have completed.
   ///
@@ -160,7 +167,7 @@ class Invoker {
     }
 
     _controller.addError(error, stackTrace);
-    _outstandingCallbacks.removeAllOutstandingCallbacks();
+    removeAllOutstandingCallbacks();
 
     // If a test was marked as success but then had an error, that indicates
     // that it was poorly-written and could be flaky.