Remove deprecated parts of dart:async.

pkg/barback/lib/src/errors.dart

Index: pkg/barback/lib/src/errors.dart
diff --git a/pkg/barback/lib/src/errors.dart b/pkg/barback/lib/src/errors.dart
index a240bb41a43e447d70a9430bc0e0f7aac5d2cde1..bb9af79d416245af60a316ddb9e9fd92a47692ae 100644
--- a/pkg/barback/lib/src/errors.dart
+++ b/pkg/barback/lib/src/errors.dart
@@ -115,15 +115,17 @@ class InvalidOutputException implements BarbackException {
 abstract class _WrappedException implements BarbackException {
   /// The wrapped exception.
   final error;
+  final StackTrace stackTrace;
 
-  _WrappedException(this.error);
+  _WrappedException(this.error, this.stackTrace);
 
   String get _message;
 
   String toString() {
     var result = "$_message: $error";
 
-    var stack = getAttachedStackTrace(error);
+    var stack = stackTrace;
+    if (stack == null && error is Error) stack = error.stackTrace;
     if (stack != null) {
       result = "$result\n${new Trace.from(stack).terse}";
     }
@@ -137,8 +139,8 @@ class TransformerException extends _WrappedException {
   /// The transform that threw the exception.
   final TransformInfo transform;
 
-  TransformerException(this.transform, error)
-      : super(error);
+  TransformerException(this.transform, error, StackTrace stackTrace)
+      : super(error, stackTrace);
 
   String get _message => "Transform $transform threw error";
 }
@@ -150,8 +152,8 @@ class TransformerException extends _WrappedException {
 class AssetLoadException extends _WrappedException {
   final AssetId id;
 
-  AssetLoadException(this.id, error)
-      : super(error);
+  AssetLoadException(this.id, error, [StackTrace stackTrace])
+      : super(error, stackTrace);
 
   String get _message => "Failed to load source asset $id";
 }