Roll Dart SDK from 253022eb1b27 to 6489a0c68df8 (4 revisions) (#18626)

diff --git a/DEPS b/DEPS
index 8b0251f..e400f22 100644
--- a/DEPS
+++ b/DEPS
@@ -34,7 +34,7 @@
   # Dart is: https://github.com/dart-lang/sdk/blob/master/DEPS.
   # You can use //tools/dart/create_updated_flutter_deps.py to produce
   # updated revision list of existing dependencies.
-  'dart_revision': '253022eb1b27bf8362c1e86c9a1b1753ef186072',
+  'dart_revision': '6489a0c68df856b2796791ba147766b70cbb5288',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py