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 b5463accb7d3beaa7ee4804895302d73d654e717..c39636efbf17e1043c19a6d6f7223b84365914a1 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)                                                  \