Preserve the stack traces for load errors in isolates and iframes.

lib/src/runner/load_exception_suite.dart

 // Copyright (c) 2015, 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.
 
 library test.runner.load_exception_suite;
 
+import 'dart:async';
+
 import '../backend/invoker.dart';
 import '../backend/metadata.dart';
 import '../backend/suite.dart';
 import 'load_exception.dart';
 
 /// A test suite generated from a load exception.
 ///
 /// Load exceptions are exposed as test suites so that they can be presented
 /// alongside successful tests.
 class LoadExceptionSuite extends Suite {
   /// The exception that this suite exposes.
   final LoadException exception;
+  final StackTrace stackTrace;
 
-  LoadExceptionSuite(LoadException exception)
+  LoadExceptionSuite(LoadException exception, stackTrace)
       : exception = exception,
+        stackTrace = stackTrace,
         super([
-          new LocalTest("load error", new Metadata(), () => throw exception)
+          new LocalTest("load error", new Metadata(),
+              () => new Future.error(exception, stackTrace))
         ], path: exception.path);
 }