Revert "Roll Dart SDK from 6ecae20 to 3cc6cda (2 revisions) (flutter#25877)"

This reverts commit 03bd2e3.

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': '3cc6cdab8eaff6127e9c68501691dcd57f975bea',
+  'dart_revision': '6ecae204598d0c3767786b96f6e759523b0a7d3a',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py