Bump dart version to 3.7

Change-Id: I9fec9765abfb46b2a3753367c20ac807fbdd487e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/388600
Commit-Queue: Reid Baker <reidbaker@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>

pkg/kernel/lib/default_language_version.dart

@@ -9,4 +9,4 @@
 
 import "ast.dart";
 
-const Version defaultLanguageVersion = const Version(3, 6);
+const Version defaultLanguageVersion = const Version(3, 7);