diff --git a/src/ast.ts b/src/ast.ts
index da2271ef3d..01d8e9a421 100644
--- a/src/ast.ts
+++ b/src/ast.ts
@@ -18,7 +18,6 @@ import {
   CommonFlags,
   PATH_DELIMITER,
   LIBRARY_PREFIX,
-  LIBRARY_SUBST
 } from "./common";
 
 import {
@@ -1668,7 +1667,7 @@ export class Source extends Node {
 
   /** Checks if this source represents native code. */
   get isNative(): bool {
-    return this.internalPath == LIBRARY_SUBST;
+    return this == Source._native;
   }
 
   /** Checks if this source is part of the (standard) library. */