Roll Dart SDK to 2.0.0-dev.16.0, along with associated package updates (#13857)

Rolling the Dart SDK to 2.0, with associated package updates.  

https://groups.google.com/forum/#!topic/flutter-dev/E22RdKAYjs8
diff --git a/examples/flutter_gallery/pubspec.yaml b/examples/flutter_gallery/pubspec.yaml
index 24e5121..856a3ad 100644
--- a/examples/flutter_gallery/pubspec.yaml
+++ b/examples/flutter_gallery/pubspec.yaml
@@ -23,7 +23,7 @@
   flutter_driver:
     sdk: flutter
 
-  args: 0.13.7 # TRANSITIVE DEPENDENCY
+  args: 1.2.0 # TRANSITIVE DEPENDENCY
   async: 1.13.3 # TRANSITIVE DEPENDENCY
   barback: 0.15.2+14 # TRANSITIVE DEPENDENCY
   boolean_selector: 1.0.2 # TRANSITIVE DEPENDENCY