Add chain support to barback.

pkg/barback/lib/src/asset_cascade.dart

Index: pkg/barback/lib/src/asset_cascade.dart
diff --git a/pkg/barback/lib/src/asset_cascade.dart b/pkg/barback/lib/src/asset_cascade.dart
index df98f1dfcc828673919ae32c021694a4290cfc7c..3454899df307c0e467193d74aa165abd342400b6 100644
--- a/pkg/barback/lib/src/asset_cascade.dart
+++ b/pkg/barback/lib/src/asset_cascade.dart
@@ -122,7 +122,7 @@ class AssetCascade {
     // Keep track of logged errors so we can know that the build failed.
     onLog.listen((entry) {
       if (entry.level == LogLevel.ERROR) {
-        // TODO(nweiz): keep track of stack trace.
+        // TODO(nweiz): keep track of stack chain.
         _accumulatedErrors.add(
             new TransformerException(entry.transform, entry.message, null));
       }