Make pkg/stack_trace use stack chains.

pkg/scheduled_test/test/metatest.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// A test library for testing test libraries? We must go deeper.
 ///
 /// Since unit testing code tends to use a lot of global state, it can be tough
 /// to test. This library manages it by running each test case in a child
 /// isolate, then reporting the results back to the parent isolate.
 library metatest;
@@ skipping 186 matching lines @@
       String uncaughtError) {
     _replyTo.send({
       "passed": passed,
       "failed": failed,
       "errors": errors,
       "uncaughtError": uncaughtError,
       "results": results.map((testCase) => {
         "description": testCase.description,
         "message": testCase.message,
         "result": testCase.result,
-        "stackTrace": testCase.stackTrace
+        "stackTrace": testCase.stackTrace.toString()
       }).toList()
     });
   }
 }