Add a [Transform.hasInput] convenience method to Barback.

pkg/barback/CHANGELOG.md

 ## 0.12.0
 
 * Add a `Transform.logger.fine` function that doesn't print its messages by
   default. When using Barback with pub in verbose mode, these messages will be
   printed.
 
+* Add a `Transform.hasInput` function that returns whether or not a given
+  secondary input exists.
+
 * `Transformer.allowedExtensions` now supports extensions containing multiple
   periods, such as `.dart.js`.
 
 * Transforms now pass their primary inputs through to the next phase by default.
   A transformer may still overwrite its primary input without causing a
   collision. If a transformer doesn't overwrite its primary input, it may cause
   it not to be passed through by calling `Transform.consumePrimary`. The primary
   input will be consumed by default if a transformer throws an error.
 
 * If an input requested with `Transform.getInput`, `Transform.readInput`, or
   `Transform.readInputAsString` cannot be found, an `AssetNotFoundException`
   will be thrown. This was always what the documentation said, but previously a
   `MissingInputException` was thrown instead.
 
 * If a transformer calls `Transform.logger.error`, the transformer will now be
   considered to have failed after it finishes running `apply()`. This means that
   its outputs will not be consumed by future transformers and its primary input
   will not be passed through to the next phase.