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

runtime/vm/service_test.cc

Index: runtime/vm/service_test.cc
diff --git a/runtime/vm/service_test.cc b/runtime/vm/service_test.cc
index 80d6ca307b4225230efe0bb4d5c76ca2929f65e9..8c16909accce1a7f91d1de2717027601f2a1dbbc 100644
--- a/runtime/vm/service_test.cc
+++ b/runtime/vm/service_test.cc
@@ -77,14 +77,12 @@ static RawArray* Eval(Dart_Handle lib, const char* expr) {
 
 
 static RawArray* EvalF(Dart_Handle lib, const char* fmt, ...) {
-  Isolate* isolate = Isolate::Current();
-
   va_list args;
   va_start(args, fmt);
   intptr_t len = OS::VSNPrint(NULL, 0, fmt, 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, fmt);
   OS::VSNPrint(buffer, (len + 1), fmt, args2);