[vm] Correct printing of bools when printing precompiler entry points

Change-Id: I80e6fdfc13d2ccf3dccf4c24d92c90b66868a81f
Reviewed-on: https://dart-review.googlesource.com/34026
Reviewed-by: Vyacheslav Egorov <vegorov@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>

runtime/vm/compiler/aot/precompiler.cc

@@ -777,7 +777,7 @@ void PrecompilerEntryPointsPrinter::Print() {
         Class::Handle(Isolate::Current()->class_table()->At(result_cid));
     DescribeClass(&writer, result_cls);
 
-    writer.PrintProperty("nullable", "false");
+    writer.PrintPropertyBool("nullable", false);
 
     writer.CloseObject();
     writer.CloseArray();