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

runtime/vm/native_api_impl.cc

Index: runtime/vm/native_api_impl.cc
===================================================================
--- runtime/vm/native_api_impl.cc	(revision 44762)
+++ runtime/vm/native_api_impl.cc	(working copy)
@@ -48,7 +48,7 @@
   }
   // Start the native port without a current isolate.
   IsolateSaver saver(Isolate::Current());
-  Isolate::SetCurrent(NULL);
+  Thread::ExitIsolate();
 
   NativeMessageHandler* nmh = new NativeMessageHandler(name, handler);
   Dart_Port port_id = PortMap::CreatePort(nmh);
@@ -60,7 +60,7 @@
 DART_EXPORT bool Dart_CloseNativePort(Dart_Port native_port_id) {
   // Close the native port without a current isolate.
   IsolateSaver saver(Isolate::Current());
-  Isolate::SetCurrent(NULL);
+  Thread::ExitIsolate();
 
   // TODO(turnidge): Check that the port is native before trying to close.
   return PortMap::ClosePort(native_port_id);