Remove fake isolate from concurrent sweeper and extend lifetime of VM threads.

runtime/vm/thread.h

 // 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/base_isolate.h"
 #include "vm/globals.h"
 #include "vm/os_thread.h"
 
 namespace dart {
 
 class CHA;
 class Isolate;
 
 // 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 exiting an isolate or an explicit call to CleanUp.
+// 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.
 class Thread {
  public:
   // The currently executing thread, or NULL if not yet initialized.
   static Thread* Current() {
     return reinterpret_cast<Thread*>(OSThread::GetThreadLocal(thread_key_));
   }
 
   // Makes the current thread enter 'isolate'. Also calls EnsureInit.
   static void EnterIsolate(Isolate* isolate);
-  // Makes the current thread exit its isolate. Also calls CleanUp.
+  // Makes the current thread exit its isolate.
   static void ExitIsolate();
 
+  // A VM thread other than the main mutator thread can enter an isolate as a
+  // "helper" to gain limited concurrent access to the isolate. One example is
+  // SweeperTask (which uses the class table, which is copy-on-write).
+  // TODO(koda): Properly synchronize heap access to expand allowed operations.
+  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.
   static void CleanUp();
 
   // Called at VM startup.
   static void InitOnce();
 
   // The topmost zone used for allocation in this thread.
   Zone* zone() {
@@ skipping 17 matching lines @@
       : isolate_(NULL) {}
 
   static void SetCurrent(Thread* current);
 
   DISALLOW_COPY_AND_ASSIGN(Thread);
 };
 
 }  // namespace dart
 
 #endif  // VM_THREAD_H_