Add proper API for creating private symbols wrt a library.

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index 3d06df2278a2a6ae290817d2d91c0804c74117b0..6df05a0ecdc8c5dd3a3b5b4ca584ae1663488a1b 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -238,7 +238,7 @@ INVISIBLE_LIST(MARK_FUNCTION)
 //   _MyClass@6b3832b. -> _MyClass
 //   _MyClass@6b3832b.named -> _MyClass.named
 //
-static RawString* IdentifierPrettyName(const String& name) {
+RawString* String::IdentifierPrettyName(const String& name) {
   if (name.Equals(Symbols::TopLevel())) {
     // Name of invisible top-level class.
     return Symbols::Empty().raw();
@@ -1485,7 +1485,7 @@ RawString* Class::UserVisibleName() const {
     default:
       if (!IsSignatureClass()) {
         const String& name = String::Handle(Name());
-        return IdentifierPrettyName(name);
+        return String::IdentifierPrettyName(name);
       } else {
         return Name();
       }
@@ -5050,7 +5050,7 @@ bool Function::HasOptimizedCode() const {
 
 RawString* Function::UserVisibleName() const {
   const String& str = String::Handle(name());
-  return IdentifierPrettyName(str);
+  return String::IdentifierPrettyName(str);
 }
 
 
@@ -5467,7 +5467,7 @@ RawField* Field::Clone(const Class& new_owner) const {
 
 RawString* Field::UserVisibleName() const {
   const String& str = String::Handle(name());
-  return IdentifierPrettyName(str);
+  return String::IdentifierPrettyName(str);
 }