Roll Dart SDK from 648d5f951915 to 39dc63439396 (1 revision) (flutter#24421)

Author: skia-flutter-autoroll

DEPS

@@ -35,7 +35,7 @@ vars = {
   # 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': '648d5f9519155d437dcdbf7b8a974836f309fa2c',
+  'dart_revision': '39dc634393968ee146c5b3ffe7d5d0680c65e7ab',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py