Remove more uses of Isolate::current_zone.

runtime/vm/handles_impl.h

Index: runtime/vm/handles_impl.h
diff --git a/runtime/vm/handles_impl.h b/runtime/vm/handles_impl.h
index 370b34e440d7dee6a03f6097477a67599f4ed1bd..ad005bb8d60172df8a37a390fbdf73bf7c9dc288 100644
--- a/runtime/vm/handles_impl.h
+++ b/runtime/vm/handles_impl.h
@@ -123,18 +123,18 @@ uword Handles<kHandleSizeInWords,
 }
 
 
-// Figure out the current zone using the current Isolate and
+// Figure out the current zone using the current Thread and
 // check if the specified handle has been allocated in this zone.
 template <int kHandleSizeInWords, int kHandlesPerChunk, int kOffsetOfRawPtr>
 bool Handles<kHandleSizeInWords,
              kHandlesPerChunk,
              kOffsetOfRawPtr>::IsZoneHandle(uword handle) {
-  // TODO(5411412): Accessing the current isolate is a performance problem,
+  // TODO(5411412): Accessing the current thread is a performance problem,
   // consider passing it down as a parameter.
-  Isolate* isolate = Isolate::Current();
-  ASSERT(isolate != NULL);
-  ASSERT(isolate->current_zone() != NULL);
-  Handles* handles = isolate->current_zone()->handles();
+  Thread* thread = Thread::Current();
+  ASSERT(thread != NULL);
+  ASSERT(thread->zone() != NULL);
+  Handles* handles = thread->zone()->handles();
   ASSERT(handles != NULL);
   return handles->IsValidZoneHandle(handle);
 }