Revert dart roll to fix Android breakage.

Reverts 2c10c3a7 manually.
See https://github.com/flutter/engine/pull/1620

DEPS

@@ -25,7 +25,7 @@ vars = {
 
   # Note: When updating the Dart revision, ensure that all entries that are
   # dependencies of dart are also updated
-  'dart_revision': '9416219c28e500a354ff0086765601efa5b1378e',
+  'dart_revision': '6aab1cecb25f8e04087320f2082336073628afb4',
   'dart_observatory_packages_revision': '5c199c5954146747f75ed127871207718dc87786',
   'dart_root_certificates_revision': 'c3a41df63afacec62fcb8135196177e35fe72f71',