Split Thread/Isolate instances, but still maintain 1:1 relationship.

runtime/vm/thread.cc

Index: runtime/vm/thread.cc
===================================================================
--- runtime/vm/thread.cc	(revision 0)
+++ runtime/vm/thread.cc	(working copy)
@@ -0,0 +1,30 @@
+// 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.
+
+#include "vm/thread.h"
+
+#include "vm/isolate.h"
+#include "vm/os_thread.h"
+
+
+namespace dart {
+
+// The single thread local key which stores all the thread local data
+// for a thread.
+// TODO(koda): Can we merge this with ThreadInterrupter::thread_state_key_?
+ThreadLocalKey Thread::thread_key = OSThread::kUnsetThreadLocalKey;
+
+
+void Thread::InitOnce() {
+  ASSERT(thread_key == OSThread::kUnsetThreadLocalKey);
+  thread_key = OSThread::CreateThreadLocal();
+  ASSERT(thread_key != OSThread::kUnsetThreadLocalKey);
+}
+
+
+void Thread::SetCurrent(Thread* current) {
+  OSThread::SetThreadLocal(thread_key, reinterpret_cast<uword>(current));
+}
+
+}  // namespace dart