Fix to asiva's code review comments on TBR Cl.

runtime/vm/mirrors_api_impl.cc

Index: runtime/vm/mirrors_api_impl.cc
diff --git a/runtime/vm/mirrors_api_impl.cc b/runtime/vm/mirrors_api_impl.cc
index af7c0963b08157b20415fe2170823592fa7dcb00..1ab23234a29d2a10fc9b3db119af60403751269b 100644
--- a/runtime/vm/mirrors_api_impl.cc
+++ b/runtime/vm/mirrors_api_impl.cc
@@ -323,20 +323,6 @@ DART_EXPORT Dart_Handle Dart_LibraryName(Dart_Handle library) {
   return Api::NewHandle(isolate, name.raw());
 }
 
-DART_EXPORT Dart_Handle Dart_LibraryId(Dart_Handle library,
-                                       intptr_t* library_id) {
-  Isolate* isolate = Isolate::Current();
-  DARTSCOPE(isolate);
-  const Library& lib = Api::UnwrapLibraryHandle(isolate, library);
-  if (lib.IsNull()) {
-    RETURN_TYPE_ERROR(isolate, library, Library);
-  }
-  if (library_id != NULL) {
-    *library_id = lib.index();
-  }
-  return Api::Success();
-}
-
 DART_EXPORT Dart_Handle Dart_LibraryGetClassNames(Dart_Handle library) {
   Isolate* isolate = Isolate::Current();
   DARTSCOPE(isolate);