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.
If a transform calls Transform.getInput, Transform.readInput, Transform.readInputAsString, or Transform.hasInput on an input that doesn't exist, the transform will be re-run if that input is created in the future.