Remove deprecated parts of dart:async.

sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart

Index: sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart b/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
index 621e85a1bb7df2446bc9f1be9c67079eeabdc9b1..2552b09aee921397d23862640ac0a701768a400f 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
@@ -379,7 +379,7 @@ class CrossIsolateException implements Exception {
   /// Serializes [error] to a map that can safely be passed across isolate
   /// boundaries.
   static Map serialize(error, [StackTrace stack]) {
-    if (stack == null) stack = getAttachedStackTrace(error);
+    if (stack == null && error is Error) stack = error.stackTrace;
     return {
       'type': error.runtimeType.toString(),
       'message': getErrorMessage(error),