Revert "Roll Dart SDK from 639cf51f4472 to 1d0b890287a4 (1 revision) (flutter#50129)"

This reverts commit af987df.

Author: zanderso

DEPS

@@ -64,7 +64,7 @@ vars = {
   # Dart is: https://github.com/dart-lang/sdk/blob/main/DEPS
   # You can use //tools/dart/create_updated_flutter_deps.py to produce
   # updated revision list of existing dependencies.
-  'dart_revision': '1d0b890287a482713ce97a9a60d39727c4e9176b',
+  'dart_revision': '639cf51f4472e4f86c5308e442b89b0a6a74347a',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py