Revert "Roll Dart SDK from 86c7493 to b4210cc (2 revisions) (flutter#26006)"

This reverts commit 3ffb8ef.

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': 'b4210cc43086e68526369ef441f26ad91ee3446e',
+  'dart_revision': '86c749398b3ace9707030dbda522e2d02aa90d34',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py