Adding an aggregate transformer example to barback. Also updating the previous examples.

pkg/barback/example/markdown_converter/pubspec.yaml

Index: pkg/barback/example/markdown_converter/pubspec.yaml
diff --git a/pkg/barback/example/markdown_converter/pubspec.yaml b/pkg/barback/example/markdown_converter/pubspec.yaml
index f212c9f57d2d5b3e8b1b3295960b81c781e24ca4..5cd04d6f186813ad8aea654e8796c161b9e5fb05 100644
--- a/pkg/barback/example/markdown_converter/pubspec.yaml
+++ b/pkg/barback/example/markdown_converter/pubspec.yaml
@@ -2,9 +2,15 @@ name: markdown_converter
 description: This hello world example implements a simple
              transformer that converts a markdown file (with
              a ".mdown", ".md", or a ".markdown" extension) to HTML.
+
 dependencies:
-  barback: any
+  barback: ">=0.14.1 <0.16.0"
   markdown: any
 
+# Override the barback dependency so this example always uses the version
+# of barback it's bundled with.
+dependency_overrides:
+  barback: {path: ../..}
+
 transformers:
 - markdown_converter