Add a --preview-port flag to dart migrate.

Change-Id: I526a2e40198960e0e3fb3509ed8a84979f520b07
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/141765
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Paul Berry <paulberry@google.com>

https://dart.googlesource.com/sdk/+/278a2db4df1a1fef349b89aeaca367dc7463bab7
diff --git a/commits.json b/commits.json
index b950e84..01cc863 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "external/github.com/flutter/engine":"767b48abc91e474ed170b573ee1262ee1088bb0e",
   "external/github.com/flutter/flutter":"0bc361f09b130b410ccd63af77665e5e0559c150",
-  "sdk":"f04053d767466f057ab5c414cee3e257ea7a8959"
+  "sdk":"278a2db4df1a1fef349b89aeaca367dc7463bab7"
 }
\ No newline at end of file