Add .disconnect() to MessageBuses

lib/bus.js

@@ -18,6 +18,24 @@ module.exports = function bus(conn, opts) {
   this.signals = new EventEmitter();
   this.exportedObjects = {};
 
+  this.disconnect = function () {
+    if (// Some of these might be redundant, but best to check everything explicitly:
+      conn.writableEnded ||
+      conn.readableEnded ||
+      conn.closed ||
+      conn.destroyed ||
+      conn.errored
+    ) {
+      return Promise.resolve();
+    }
+
+    return new Promise(function (resolve, reject) {
+      conn.on('end', resolve);
+      conn.on('error', reject);
+      conn.end();
+    });
+  };
+
   this.invoke = promisify(function(msg, callback) {
     if (!msg.type) msg.type = constants.messageType.methodCall;
     msg.serial = self.serial++;

types.d.ts

@@ -8,6 +8,7 @@ export function createClient(options: {
 
 export interface DBusClient {
     getService(name: string): DBusService;
+    disconnect(): Promise<void>;
 }
 
 export interface DBusService {