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

runtime/vm/json_stream.cc

Index: runtime/vm/json_stream.cc
diff --git a/runtime/vm/json_stream.cc b/runtime/vm/json_stream.cc
index e716e508e45381b6ed937406b35b3926ad852ad0..5b10ab0f6803fca90299bd0a3f50e88e30a88404 100644
--- a/runtime/vm/json_stream.cc
+++ b/runtime/vm/json_stream.cc
@@ -131,14 +131,12 @@ void JSONStream::PrintError(intptr_t code,
     JSONObject data(&jsobj, "data");
     PrintRequest(&data, this);
     if (details_format != NULL) {
-      Isolate* isolate = Isolate::Current();
-
       va_list args;
       va_start(args, details_format);
       intptr_t len = OS::VSNPrint(NULL, 0, details_format, args);
       va_end(args);
 
-      char* buffer = isolate->current_zone()->Alloc<char>(len + 1);
+      char* buffer = Thread::Current()->zone()->Alloc<char>(len + 1);
       va_list args2;
       va_start(args2, details_format);
       OS::VSNPrint(buffer, (len + 1), details_format, args2);