Add a composite index for flutter repo and creation date on Checklist. (#1153)

Author: yusufm

app_dart/index.yaml

@@ -13,6 +13,12 @@ indexes:
   - name: Branch
   - name: CreateTimestamp
     direction: desc
+- kind: Checklist
+  properties:
+  - name: FlutterRepositoryPath
+  - name: Branch
+  - name: CreateTimestamp
+    direction: desc
 - kind: Task
   ancestor: yes
   properties: