Mark flutter_gallery__preview_dart_2_bulid unflaky (flutter#13170)

This was fixed in flutter#13150.

That commit rolled the engine to:
  flutter/engine@93b2179,

which rolled Dart to:
  dart-lang/sdk@70e5dea

which includes:
  dart-lang/sdk@d38c08d

Which fixes the breakage introduced in flutter#13134.

This reverts commit f5d9c77.

dev/devicelab/manifest.yaml

@@ -338,7 +338,6 @@ tasks:
       Gallery under --preview-dart-2 that enables new Dart 2.0 frontend.
     stage: devicelab
     required_agent_capabilities: ["linux/android"]
-    flaky: true
 
   flutter_gallery__start_up:
     description: >