Add proper API for creating private symbols wrt a library.

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index a1272a711c91598a24e1fdb0c6143e5b143a592e..00d222e13e346ee5d3e37e1ff60d525d91de8f6b 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -4792,6 +4792,9 @@ class String : public Instance {
   static RawString* ToLowerCase(const String& str,
                                 Heap::Space space = Heap::kNew);
 
+  static RawString* IdentifierPrettyName(const String& name);
+  static RawString* IdentifierPrettyNameRetainPrivate(const String& name);
+
   static bool EqualsIgnoringPrivateKey(const String& str1,
                                        const String& str2);