Remove deprecated parts of dart:async.

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 cc9056012be4b7c2dd79ef0ca941970f889ef24e..5b409385c80e6a02c45f169d35a1496a070659cb 100644
--- a/pkg/barback/lib/src/asset_cascade.dart
+++ b/pkg/barback/lib/src/asset_cascade.dart
@@ -122,8 +122,9 @@ 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.
         _accumulatedErrors.add(
-            new TransformerException(entry.transform, entry.message));
+            new TransformerException(entry.transform, entry.message, null));
       }
     });
   }
@@ -187,8 +188,8 @@ class AssetCascade {
         _loadingSources.remove(id);
       }).then((asset) {
         var controller = _sourceControllerMap[id].setAvailable(asset);
-      }).catchError((error) {
-        reportError(new AssetLoadException(id, error));
+      }).catchError((error, stack) {
+        reportError(new AssetLoadException(id, error, stack));
 
         // TODO(nweiz): propagate error information through asset nodes.
         _sourceControllerMap.remove(id).setRemoved();