Remove deprecated parts of dart:async.

pkg/barback/lib/src/build_result.dart

Index: pkg/barback/lib/src/build_result.dart
diff --git a/pkg/barback/lib/src/build_result.dart b/pkg/barback/lib/src/build_result.dart
index 65e82690ccc9f9ea23015fc44b745b61b650db47..52b341e5af4615f51ce552f19ba635f781ffdf91 100644
--- a/pkg/barback/lib/src/build_result.dart
+++ b/pkg/barback/lib/src/build_result.dart
@@ -47,8 +47,10 @@ class BuildResult {
     if (succeeded) return "success";
 
     return "errors:\n" + errors.map((error) {
-      var stackTrace = getAttachedStackTrace(error);
-      if (stackTrace != null) stackTrace = new Trace.from(stackTrace);
+      var stackTrace = null;
+      if (error is TransformerException || error is AssetLoadException) {
+        stackTrace = new Trace.from(error.stackTrace);
+      }
 
       var msg = new StringBuffer();
       msg.write(prefixLines(error.toString()));