Mark pkg/analyzer_plugin/test/plugin/folding_mixin_test as Slow
Mark pkg/analyzer_plugin/test/plugin/folding_mixin_test as Slow

The bot had several timeouts during the day.  It may be similar to what
was fixed in
https://github.com/dart-lang/sdk/commit/5263d17f7480ff0648fa319ba1a8ed62d5116111
Change-Id: Ife4a1e6d2b40992d260ae03287d844ee9cceb79b
Reviewed-on: https://dart-review.googlesource.com/8600
Reviewed-by: Karl Klose <49bc0b76cffb0b10cd8c84f9cabc97a31ffe6343@google.com>

dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk