Add chain support to barback.

pkg/barback/lib/src/serialize.dart

Index: pkg/barback/lib/src/serialize.dart
diff --git a/pkg/barback/lib/src/serialize.dart b/pkg/barback/lib/src/serialize.dart
index fa5adeaba5d2f1fb069004e376ad1aabcf2ca314..a9b3d6afda8a27e61dd118b2b0dfad4219f02ffa 100644
--- a/pkg/barback/lib/src/serialize.dart
+++ b/pkg/barback/lib/src/serialize.dart
@@ -88,8 +88,8 @@ class CrossIsolateException implements Exception {
   /// property.
   final String message;
 
-  /// The exception's stack trace, or `null` if no stack trace was available.
-  final Trace stackTrace;
+  /// The exception's stack chain, or `null` if no stack chain was available.
+  final Chain stackTrace;
 
   /// Loads a [CrossIsolateException] from a serialized representation.
   ///
@@ -98,7 +98,7 @@ class CrossIsolateException implements Exception {
     var type = error['type'];
     var message = error['message'];
     var stackTrace = error['stack'] == null ? null :
-            new Trace.parse(error['stack']);
+            new Chain.parse(error['stack']);
     return new CrossIsolateException._(type, message, stackTrace);
   }
 
@@ -114,7 +114,7 @@ class CrossIsolateException implements Exception {
     return {
       'type': error.runtimeType.toString(),
       'message': _getErrorMessage(error),
-      'stack': stack == null ? null : stack.toString()
+      'stack': stack == null ? null : new Chain.forTrace(stack).toString()
     };
   }