Revert analyzer Dart language version to 2.12

Change-Id: I2076c6134f8da11ef78db96433241e2eea779a15
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/204700
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>

https://dart.googlesource.com/sdk/+/0740566438c510835b87cad99f088c7302c5ed5d
diff --git a/commits.json b/commits.json
index 1584aa8..643583c 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "external/github.com/flutter/engine":"98253370f53d81e8e9128a8d90666771a74bdc68",
   "external/github.com/flutter/flutter":"b30c721e4c3ad79fc474ac1f91dac5f2becc5938",
-  "sdk":"a98d1da827b092f5b1ef602cef3820d09efc0469"
+  "sdk":"0740566438c510835b87cad99f088c7302c5ed5d"
 }
\ No newline at end of file