Clean up the test srapper code so that it does not use reportTestError, and made that private.

lib/unittest/config.dart

 // Copyright (c) 2011, 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.
 
 /** This file is sourced by unitest.dart. */
 
 /**
  * Hooks to configure the unittest library for different platforms. This class
  * implements the API in a platform-independent way. Tests that want to take
  * advantage of the platform can create a subclass and override methods from
@@ skipping 111 matching lines @@
     // An exception is used by the test infrastructure to detect failure.
     if (!success) throw new Exception("Some tests failed.");
   }
 
   String _indent(String str) {
     // TODO(nweiz): Use this simpler code once issue 2980 is fixed.
     // return str.replaceAll(const RegExp("^", multiLine: true), "  ");
 
     return Strings.join(str.split("\n").map((line) => "  $line"), "\n");
   }
+
+  /** Handle errors that happen outside the tests. */
+  // TODO(vsm): figure out how to expose the stack trace here
+  // Currently e.message works in dartium, but not in dartc.
+  handleExternalError(e, String message) =>
+      _reportTestError('$message\nCaught $e', '');
 }