refactor(flatten-threads): migrate to the new DI architecture

Bug: twpowertools:176
Change-Id: I7a24fb504ce53697112f11128d2d5249a5a7c7e7
diff --git a/src/presentation/nodeWatcher/NodeWatcherHandler.ts b/src/presentation/nodeWatcher/NodeWatcherHandler.ts
index b6c3a25..67801f4 100644
--- a/src/presentation/nodeWatcher/NodeWatcherHandler.ts
+++ b/src/presentation/nodeWatcher/NodeWatcherHandler.ts
@@ -13,11 +13,11 @@
   RemovedNode,
 }
 
-export interface NodeMutation {
+export interface NodeMutation<T extends Node = Node> {
   /**
    * Node being mutated.
    */
-  node: Node;
+  node: T;
   /**
    * Which mutation has occurred to the node.
    */