Make pkg/stack_trace use stack chains.

pkg/scheduled_test/lib/src/descriptor/pattern_descriptor.dart

Index: pkg/scheduled_test/lib/src/descriptor/pattern_descriptor.dart
diff --git a/pkg/scheduled_test/lib/src/descriptor/pattern_descriptor.dart b/pkg/scheduled_test/lib/src/descriptor/pattern_descriptor.dart
index 130cfddcf13a0b55c5f2498a16d5d1e78cfc8e59..4ce8602fb6f2256cb332c6c42780de5bca47bf63 100644
--- a/pkg/scheduled_test/lib/src/descriptor/pattern_descriptor.dart
+++ b/pkg/scheduled_test/lib/src/descriptor/pattern_descriptor.dart
@@ -8,6 +8,7 @@ import 'dart:async';
 import 'dart:io';
 
 import 'package:path/path.dart' as path;
+import 'package:stack_trace/stack_trace.dart';
 
 import '../../descriptor.dart';
 import '../../scheduled_test.dart';
@@ -112,5 +113,6 @@ class PatternDescriptor extends Descriptor {
   }
 
   Future create([String parent]) => new Future.error(
-      new UnsupportedError("Pattern descriptors don't support create()."));
+      new UnsupportedError("Pattern descriptors don't support create()."),
+      new Chain.current());
 }