[autoroller] Roll from dart 'dev' branch, rather than from 'master'

This is to provide more robust version reporting of dart sdk built in flutter.

Change-Id: I854a644fc603e12ad90487caf31a27a563b278f0
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/307783
Reviewed-by: Ravi Mistry <rmistry@google.com>
Reviewed-by: Eric Boren <borenet@google.com>
Commit-Queue: Ravi Mistry <rmistry@google.com>
diff --git a/autoroll/config/dart-sdk-flutter-engine.json b/autoroll/config/dart-sdk-flutter-engine.json
index 8d2aded..bb5f739 100644
--- a/autoroll/config/dart-sdk-flutter-engine.json
+++ b/autoroll/config/dart-sdk-flutter-engine.json
@@ -23,7 +23,7 @@
     "checksWaitFor": ["luci-engine"]
   },
   "githubDEPSRepoManager": {
-    "childBranch": "master",
+    "childBranch": "dev",
     "childPath": "src/third_party/dart",
     "childRepo": "https://dart.googlesource.com/sdk.git",
     "childRevLinkTmpl": "https://dart.googlesource.com/sdk.git/+show/%s",