Introduce simple Thread class to support gradual refactoring of interfaces.

runtime/vm/thread.h

Index: runtime/vm/thread.h
===================================================================
--- runtime/vm/thread.h	(revision 0)
+++ runtime/vm/thread.h	(working copy)
@@ -0,0 +1,40 @@
+// 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.
+
+#ifndef VM_THREAD_H_
+#define VM_THREAD_H_
+
+#include "vm/isolate.h"
+
+namespace dart {
+
+// A VM thread; may be executing Dart code or performing helper tasks like
+// garbage collection.
+class Thread {
+ public:
+  static Thread* Current() {
+    // For now, there is still just one thread per isolate, and the Thread
+    // class just aliases the Isolate*. Once all interfaces and uses have been
+    // updated to distinguish between isolates and threads, Thread will get its
+    // own thread-local storage key.
+    return reinterpret_cast<Thread*>(Isolate::Current());
+  }
+
+  // The topmost zone used for allocation in this thread.
+  Zone* zone() {
+    return reinterpret_cast<BaseIsolate*>(this)->current_zone();
+  }
+
+  // The isolate that this thread is operating on.
+  Isolate* isolate() {
+    return reinterpret_cast<Isolate*>(this);
+  }
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(Thread);
+};
+
+}  // namespace dart
+
+#endif  // VM_THREAD_H_