Automatic thread cleanup on non-Windows platforms.

runtime/vm/thread.h

Index: runtime/vm/thread.h
===================================================================
--- runtime/vm/thread.h	(revision 45815)
+++ runtime/vm/thread.h	(working copy)
@@ -16,13 +16,9 @@
 
 // A VM thread; may be executing Dart code or performing helper tasks like
 // garbage collection or compilation. The Thread structure associated with
-// a thread is allocated when the thread enters an isolate, and destroyed
-// upon an explicit call to CleanUp.
-//
-// NOTE: When the underlying OS thread is started by the embedder, i.e., not
-// part of a ThreadPool managed by the VM, we currently leak the Thread
-// structure after its last isolate exit. We could add an explicit cleanup API,
-// or, on platforms that support it, register a cleanup callback.
+// a thread is allocated by EnsureInit before entering an isolate, and destroyed
+// automatically when the underlying OS thread exits. NOTE: On Windows, CleanUp
+// must currently be called manually (issue 23474).
 class Thread {
  public:
   // The currently executing thread, or NULL if not yet initialized.
@@ -30,7 +26,10 @@
     return reinterpret_cast<Thread*>(OSThread::GetThreadLocal(thread_key_));
   }
 
-  // Makes the current thread enter 'isolate'. Also calls EnsureInit.
+  // Initializes the current thread as a VM thread, if not already done.
+  static void EnsureInit();
+
+  // Makes the current thread enter 'isolate'.
   static void EnterIsolate(Isolate* isolate);
   // Makes the current thread exit its isolate.
   static void ExitIsolate();
@@ -42,10 +41,10 @@
   static void EnterIsolateAsHelper(Isolate* isolate);
   static void ExitIsolateAsHelper();
 
-  // Initializes the current thread as a VM thread, if not already done.
-  static void EnsureInit();
-  // Clears the state of the current thread.
+#if defined(TARGET_OS_WINDOWS)
+  // Clears the state of the current thread and frees the allocation.
   static void CleanUp();
+#endif
 
   // Called at VM startup.
   static void InitOnce();