Wrap v8natives.js into a function.

src/mirror-debugger.js

Index: src/mirror-debugger.js
diff --git a/src/mirror-debugger.js b/src/mirror-debugger.js
index 87a1febaa962225801467e8f47177ef5c30b7856..b293ba0a92dd774327d5c58672155280b5979d4d 100644
--- a/src/mirror-debugger.js
+++ b/src/mirror-debugger.js
@@ -1008,7 +1008,7 @@ FunctionMirror.prototype.source = function() {
   // Return source if function is resolved. Otherwise just fall through to
   // return undefined.
   if (this.resolved()) {
-    return builtins.FunctionSourceString(this.value_);
+    return builtins.$functionSourceString(this.value_);
   }
 };