Revert "Roll Dart SDK from 8c109a7 to b98a1ee (1 revision) (flutter#25873)"

This reverts commit e623c09.

Author: zanderso

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': 'b98a1eec5eb5727f6d211b08daf5c2b5fe27d072',
+  'dart_revision': '8c109a734bdc8ad23ac73ae5edf1e42e9c5801b1',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py