Refactor Thread lifecycle interface, add Thread::Enter/ExitIsolate.

runtime/vm/thread.h

Index: runtime/vm/thread.h
===================================================================
--- runtime/vm/thread.h	(revision 44762)
+++ runtime/vm/thread.h	(working copy)
@@ -15,20 +15,29 @@
 class Isolate;
 
 // A VM thread; may be executing Dart code or performing helper tasks like
-// garbage collection or compilation.
+// 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.
 class Thread {
  public:
-  explicit Thread(Isolate* isolate)
-      : isolate_(isolate),
-        cha_(NULL) {}
-
-  static void InitOnce();
-
+  // The currently executing thread, or NULL if not yet initialized.
   static Thread* Current() {
     return reinterpret_cast<Thread*>(OSThread::GetThreadLocal(thread_key_));
   }
-  static void SetCurrent(Thread* current);
 
+  // Makes the current thread enter 'isolate'. Also calls EnsureInit.
+  static void EnterIsolate(Isolate* isolate);
+  // Makes the current thread exit its isolate. Also calls CleanUp.
+  static void ExitIsolate();
+
+  // 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() {
     return reinterpret_cast<BaseIsolate*>(isolate())->current_zone();
@@ -47,6 +56,12 @@
   Isolate* isolate_;
   CHA* cha_;
 
+  Thread()
+      : isolate_(NULL),
+        cha_(NULL) {}
+
+  static void SetCurrent(Thread* current);
+
   DISALLOW_COPY_AND_ASSIGN(Thread);
 };