Don't warn if a transformer declares outputs that it doesn't emit.

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.
+
 ## 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.
 
 * `DeclaringTransform` no longer supports `getInput`, `readInput`,
   `readInputAsString`, or `hasInput`.
 
@@ skipping 22 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.