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

runtime/vm/allocation.cc

Index: runtime/vm/allocation.cc
diff --git a/runtime/vm/allocation.cc b/runtime/vm/allocation.cc
index b8f9cb12ae3fdbc46d2952e666e29683ccc93a65..b6ca73e10ff4c731729644ebfa673d682c6eafd8 100644
--- a/runtime/vm/allocation.cc
+++ b/runtime/vm/allocation.cc
@@ -20,12 +20,12 @@ static void* Allocate(uword size, Zone* zone) {
 
 
 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);
 }