Only test optimizing compilation in helper thread compilation test.

runtime/vm/symbols.cc

Index: runtime/vm/symbols.cc
diff --git a/runtime/vm/symbols.cc b/runtime/vm/symbols.cc
index 7cafda2542bb1e98851bb50d4ea94525b2fc0fe7..dbaba441704fcd4fcacf94251b3149d705b25431 100644
--- a/runtime/vm/symbols.cc
+++ b/runtime/vm/symbols.cc
@@ -386,8 +386,9 @@ RawString* Symbols::FromConcat(const String& str1, const String& str2) {
 // StringType can be StringSlice, ConcatString, or {Latin1,UTF16,UTF32}Array.
 template<typename StringType>
 RawString* Symbols::NewSymbol(const StringType& str) {
-  Isolate* isolate = Isolate::Current();
-  String& symbol = String::Handle(isolate);
+  Thread* thread = Thread::Current();
+  Isolate* isolate = thread->isolate();
+  String& symbol = String::Handle(thread->zone());
   {
     Isolate* vm_isolate = Dart::vm_isolate();
     SymbolTable table(isolate, vm_isolate->object_store()->symbol_table());