Finish release of 0.0.19, start dev version of 0.0.20 (#13370)

VERSION

@@ -6,4 +6,4 @@
 # incompatible way, this version number might not change. Instead, the version
 # number for package:flutter will update to reflect that change.
 
-0.0.19
+0.0.20-dev

packages/flutter/pubspec.yaml

 name: flutter
-version: 0.0.39
+version: 0.0.40-dev
 author: Flutter Authors <flutter-dev@googlegroups.com>
 description: A framework for writing Flutter applications
 homepage: http://flutter.io

packages/flutter_driver/pubspec.yaml

 name: flutter_driver
-version: 0.0.17
+version: 0.0.18-dev
 description: Integration and performance test API for Flutter applications
 homepage: http://flutter.io
 author: Flutter Authors <flutter-dev@googlegroups.com>

packages/flutter_test/pubspec.yaml

 name: flutter_test
-version: 0.0.17
+version: 0.0.18-dev
 dependencies:
   # To update these, use "flutter update-packages --force-upgrade".