Bug 1213620 - Expose DownloadsButton to the global scope for addon-compat. r=gijs

browser/components/downloads/content/indicator.js

@@ -181,6 +181,12 @@ const DownloadsButton = {
   }
 };
 
+Object.defineProperty(this, "DownloadsButton", {
+  value: DownloadsButton,
+  enumerable: true,
+  writable: false
+});
+
 ////////////////////////////////////////////////////////////////////////////////
 //// DownloadsIndicatorView