function _undoPriorUserKeyBindings() {
        _.forEach(_customKeyMapCache, function (commandID, key) {
            var normalizedKey  = normalizeKeyDescriptorString(key),
                defaults       = _.find(_.toArray(_defaultKeyMap), { "commandID": commandID }),
                defaultCommand = _defaultKeyMap[normalizedKey];

            // We didn't modified this before, so skip it.
            if (_isSpecialCommand(commandID) ||
                    _isReservedShortcuts(normalizedKey)) {
                return;
            }

            if (_isKeyAssigned(normalizedKey) &&
                    _customKeyMap[key] !== commandID && _customKeyMap[normalizedKey] !== commandID) {
                // Unassign the key from any command. e.g. "Cmd-W": "file.open" in _customKeyMapCache
                // will require us to remove Cmd-W shortcut from file.open command.
                removeBinding(normalizedKey);
            }

            // Reassign the default key binding. e.g. "Cmd-W": "file.open" in _customKeyMapCache
            // will require us to reassign Cmd-O shortcut to file.open command.
            if (defaults) {
                addBinding(commandID, defaults, brackets.platform);
            }

            // Reassign the default key binding of the previously modified command.
            // e.g. "Cmd-W": "file.open" in _customKeyMapCache will require us to reassign Cmd-W
            // shortcut to file.close command.
            if (defaultCommand && defaultCommand.key) {
                addBinding(defaultCommand.commandID, defaultCommand.key, brackets.platform);
            }
        });
    }
@private

Restores the default key bindings for all the commands that are modified by each key binding
specified in _customKeyMapCache (old version) but no longer specified in _customKeyMap (new version).