[gnome-shell] params: Fix regression

[Florian Müllner <fmuellner src gnome org>]

commit 8a5de327bb59850ad914dcceb683e275b577d83d
Author: Florian Müllner <fmuellner gnome org>
Date:   Fri Jul 5 18:10:56 2019 +0200

    params: Fix regression
    
    The first parameter to Object.assign() is the same target object that
    will be returned. That is, since commit 46874eed0 Params.parse() modifies
    the @defaults object. Usually we pass that parameter as an object literal
    and this isn't an issue, but the change breaks spectacularly in the few
    cases where we use a re-usable variable.
    
    Restore the previous behavior by copying the object first.
    
    https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/615

 js/misc/params.js | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)
---
diff --git a/js/misc/params.js b/js/misc/params.js
index 0e3af9532..6878395e9 100644
--- a/js/misc/params.js
+++ b/js/misc/params.js
@@ -1,5 +1,7 @@
 // -*- mode: js; js-indent-level: 4; indent-tabs-mode: nil -*-
 
+const Lang = imports.lang;
+
 // parse:
 // @params: caller-provided parameter object, or %null
 // @defaults-provided defaults object
@@ -21,5 +23,7 @@ function parse(params = {}, defaults, allowExtras) {
                 throw new Error(`Unrecognized parameter "${prop}"`);
     }
 
-    return Object.assign(defaults, params);
+    let defaultsCopy = {};
+    Lang.copyProperties(defaults, defaultsCopy);
+    return Object.assign(defaultsCopy, params);
 }