Make constructor closure functions invisible on the stack

Matches implicit closures and method extractor functions.

BUG=

Review URL: https://codereview.chromium.org//1282633002 .
diff --git a/runtime/vm/parser.cc b/runtime/vm/parser.cc
index 116388b..419cd4e 100644
--- a/runtime/vm/parser.cc
+++ b/runtime/vm/parser.cc
@@ -12884,6 +12884,7 @@
                                          token_pos);
   closure.set_is_generated_body(true);
   closure.set_is_debuggable(false);
+  closure.set_is_visible(false);
   closure.set_result_type(AbstractType::Handle(Type::DynamicType()));
   AddFormalParamsToFunction(&params, closure);