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

lib/unittest/config.dart

Index: lib/unittest/config.dart
===================================================================
--- lib/unittest/config.dart	(revision 10299)
+++ lib/unittest/config.dart	(working copy)
@@ -129,4 +129,10 @@
 
     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', '');
 }