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

runtime/vm/allocation.cc

 // Copyright (c) 2012, 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 "vm/allocation.h"
 
 #include "platform/assert.h"
 #include "vm/isolate.h"
 #include "vm/zone.h"
 
 namespace dart {
 
 static void* Allocate(uword size, Zone* zone) {
   ASSERT(zone != NULL);
   if (size > static_cast<uword>(kIntptrMax)) {
     FATAL1("ZoneAllocated object has unexpectedly large size %" Pu "", size);
   }
   return reinterpret_cast<void*>(zone->AllocUnsafe(size));
 }
 
 
 void* ZoneAllocated::operator new(uword size) {
-  return Allocate(size, Isolate::Current()->current_zone());
+  return Allocate(size, Thread::Current()->zone());
 }
 
 
 void* ZoneAllocated::operator new(uword size, Zone* zone) {
-  ASSERT(zone == Isolate::Current()->current_zone());
+  ASSERT(zone == Thread::Current()->zone());
   return Allocate(size, zone);
 }
 
 
 void StackResource::UnwindAbove(Isolate* isolate, StackResource* new_top) {
   StackResource* current_resource = isolate->top_resource();
   while (current_resource != new_top) {
     current_resource->~StackResource();
     current_resource = isolate->top_resource();
   }
 }
 
 }  // namespace dart