Make the "sticky error" an Error instead of a String.

runtime/vm/longjump_test.cc

 // 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.
 
 #include "vm/isolate.h"
 #include "vm/longjump.h"
 #include "vm/unit_test.h"
 
 namespace dart {
 
 static void LongJumpHelper(LongJump* jump) {
-  jump->Jump(1, "LongJump Test");
+  const Error& error =
+      Error::Handle(LanguageError::New(
+          String::Handle(String::New("LongJumpHelper"))));
+  jump->Jump(1, error);
   UNREACHABLE();
 }
 
 
 TEST_CASE(LongJump) {
   LongJump jump;
   if (setjmp(*jump.Set()) == 0) {
     LongJumpHelper(&jump);
     UNREACHABLE();
   }
 }
 
 }  // namespace dart