Roll Dart SDK from 551af75f42c0 to a70c4b0fafc2 (1 revision) (#26317)

diff --git a/DEPS b/DEPS
index a933ab3..6036c3d 100644
--- a/DEPS
+++ b/DEPS
@@ -35,7 +35,7 @@
   # 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': '551af75f42c03cdff9c1a14a889bb216fc3d4396',
+  'dart_revision': 'a70c4b0fafc207cf8413518335ed214f0ad95c6f',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py