Migrate LongJumpScope to Thread.

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/thread.h"
 #include "vm/unit_test.h"
 
 namespace dart {
 
 static void LongJumpHelper(LongJumpScope* jump) {
   const Error& error =
       Error::Handle(LanguageError::New(
           String::Handle(String::New("LongJumpHelper"))));
   jump->Jump(1, error);
   UNREACHABLE();
 }
 
 
 TEST_CASE(LongJump) {
-  LongJumpScope* base = Isolate::Current()->long_jump_base();
+  LongJumpScope* base = Thread::Current()->long_jump_base();
   {
     LongJumpScope jump;
     if (setjmp(*jump.Set()) == 0) {
       LongJumpHelper(&jump);
       UNREACHABLE();
     }
   }
-  ASSERT(base == Isolate::Current()->long_jump_base());
+  ASSERT(base == Thread::Current()->long_jump_base());
 }
 
 }  // namespace dart