Fix wrong initializer order -- not caught on mac build because mac

runtime/vm/thread_pool.h

Index: runtime/vm/thread_pool.h
===================================================================
--- runtime/vm/thread_pool.h	(revision 5484)
+++ runtime/vm/thread_pool.h	(working copy)
@@ -77,8 +77,8 @@
     // Fields owned by ThreadPool.  Workers should not look at these
     // directly.  It's like looking at the sun.
     bool owned_;         // Protected by ThreadPool::mutex_
+    Worker* all_next_;   // Protected by ThreadPool::mutex_
     Worker* idle_next_;  // Protected by ThreadPool::mutex_
-    Worker* all_next_;   // Protected by ThreadPool::mutex_
 
     DISALLOW_COPY_AND_ASSIGN(Worker);
   };