Stop running pub get at gclient sync time and fix build bugs

mojo/public/dart/third_party/barback/example/simple_transformer/pubspec.yaml

+name: simple_transformer
+description: This hello world example implements a simple
+             transformer that inserts a copyright string into
+             an input asset - a file that ends with ".txt".
+
+dependencies:
+  barback: ">=0.14.1 <0.16.0"
+
+# Override the barback dependency so this example always uses the version
+# of barback it's bundled with.
+dependency_overrides:
+  barback: {path: ../..}
+
+transformers:
+- simple_transformer