Fixed flutter create's "package" template's error in pubspec.yaml. (#14977)

packages/flutter_tools/templates/package/pubspec.yaml.tmpl

@@ -11,6 +11,10 @@ dependencies:
 dev_dependencies:
   flutter_test:
     sdk: flutter
+  {{#withDriverTest}}
+  flutter_driver:
+    sdk: flutter
+  {{/withDriverTest}}
 
 # For information on the generic Dart part of this file, see the
 # following page: https://www.dartlang.org/tools/pub/pubspec
@@ -47,4 +51,4 @@ flutter:
   #         weight: 700
   #
   # For details regarding fonts in packages, see
-  # https://flutter.io/custom-fonts/#from-packages
\ No newline at end of file
+  # https://flutter.io/custom-fonts/#from-packages