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

Bug: twpowertools:176
Change-Id: I7a24fb504ce53697112f11128d2d5249a5a7c7e7
diff --git a/src/features/flattenThreads/presentation/nodeWatcherHandlers/replyBtn.handler.ts b/src/features/flattenThreads/presentation/nodeWatcherHandlers/replyBtn.handler.ts
new file mode 100644
index 0000000..66eff17
--- /dev/null
+++ b/src/features/flattenThreads/presentation/nodeWatcherHandlers/replyBtn.handler.ts
@@ -0,0 +1,18 @@
+import CssSelectorNodeWatcherHandler from '../../../../infrastructure/presentation/nodeWatcher/handlers/CssSelectorHandler.adapter';
+import { NodeMutation } from '../../../../presentation/nodeWatcher/NodeWatcherHandler';
+import FlattenThreads, {
+  kReplyActionButtonsSelector,
+} from '../../core/flattenThreads';
+
+/** Inject reply button in non-nested view */
+export default class FlattenThreadsReplyBtnHandler extends CssSelectorNodeWatcherHandler {
+  cssSelector = kReplyActionButtonsSelector;
+
+  constructor(private flattenThreads: FlattenThreads) {
+    super();
+  }
+
+  onMutatedNode({ node }: NodeMutation) {
+    this.flattenThreads.injectReplyBtnIfApplicable(node);
+  }
+}