Don't require Transformer methods to return Futures.

pkg/barback/CHANGELOG.md

 ## 0.13.1
 
 * Only run `Transformer.isPrimary` once for each asset.
 
 * Don't warn if a lazy or declaring transformer doesn't emit outputs that it has
   declared. This is valid for transformers like dart2js that need to read their
   primary input in order to determine whether they should run.
 
+* Allow `Transformer.isPrimary`, `Transformer.apply`, and
+  `DeclaringTransformer.declareOutputs` to return non-`Future` values if they
+  run synchronously.
+
 * Fix a deadlock bug when a lazy primary input to a lazy transformer became
   dirty while the transformer's `apply` method was running.
 
 ## 0.13.0
 
 * `Transformer.isPrimary` now takes an `AssetId` rather than an `Asset`.
 
 * `DeclaringTransform` now only exposes the primary input's `AssetId`, rather
   than the primary `Asset` object.
 
@@ skipping 25 matching lines @@
 
 * 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.