Migrate most uses of Isolate::current_zone to Thread::zone.

runtime/vm/timeline.cc

 // 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.
 
 #include <cstdlib>
 
 #include "vm/isolate.h"
 #include "vm/json_stream.h"
 #include "vm/object.h"
 #include "vm/thread.h"
@@ skipping 353 matching lines @@
     return;
   }
 
   JSONStream js;
   PrintJSON(&js);
 
   const char* format = "%s/dart-timeline-%" Pd "-%" Pd ".json";
   intptr_t pid = OS::ProcessId();
   intptr_t len = OS::SNPrint(NULL, 0, format,
                              directory, pid, isolate->main_port());
-  char* filename = isolate->current_zone()->Alloc<char>(len + 1);
+  char* filename = Thread::Current()->zone()->Alloc<char>(len + 1);
   OS::SNPrint(filename, len + 1, format,
               directory, pid, isolate->main_port());
   void* file = (*file_open)(filename, true);
   if (file == NULL) {
     OS::Print("Failed to write timeline file: %s\n", filename);
     return;
   }
   (*file_write)(js.buffer()->buf(), js.buffer()->length(), file);
   (*file_close)(file);
 }
@@ skipping 123 matching lines @@
   return event;
 }
 
 
 void TimelineEventStreamingRecorder::CompleteEvent(TimelineEvent* event) {
   StreamEvent(event);
   delete event;
 }
 
 }  // namespace dart