Migrator: use pub.bat on Windows

Fixes https://github.com/dart-lang/sdk/issues/42102

Change-Id: Ic2dd785a84835402c7af3ef7b8d15285585a0afb
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/149386
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Mike Fairhurst <mfairhurst@google.com>
Reviewed-by: Janice Collins <jcollins@google.com>

https://dart.googlesource.com/sdk/+/2dea325d871132e84eb9d624ebf7f0fff95c3398
diff --git a/commits.json b/commits.json
index 8aedc6d..da7825a 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "external/github.com/flutter/engine":"c5d012900f7db0dc5dc9afb46f76d16b87915db0",
   "external/github.com/flutter/flutter":"e853155c7e36264514970dd6b67eb110fd394f78",
-  "sdk":"f049a410e5de6fbf6745606cf59735047bfadf4a"
+  "sdk":"2dea325d871132e84eb9d624ebf7f0fff95c3398"
 }
\ No newline at end of file