Add proper API for creating private symbols wrt a library.

runtime/vm/bootstrap_natives.h

Index: runtime/vm/bootstrap_natives.h
diff --git a/runtime/vm/bootstrap_natives.h b/runtime/vm/bootstrap_natives.h
index f4b947ce0c4af8ee453073d386b28916dab445eb..d654818cd4e6f66f0cca6ab4ceb74a5ea4607024 100644
--- a/runtime/vm/bootstrap_natives.h
+++ b/runtime/vm/bootstrap_natives.h
@@ -255,6 +255,8 @@ namespace dart {
   V(Mirrors_makeLocalClassMirror, 1)                                           \
   V(Mirrors_makeLocalTypeMirror, 1)                                            \
   V(Mirrors_makeLocalMirrorSystem, 0)                                          \
+  V(Mirrors_mangleName, 2)                                                     \
+  V(Mirrors_unmangleName, 1)                                                   \
   V(MirrorReference_equals, 2)                                                 \
   V(InstanceMirror_identityHash, 1)                                            \
   V(InstanceMirror_invoke, 5)                                                  \