Mojo JS Bindings: restore the ServiceProvider class, add an application test

services/js/test/echo.js

+#!mojo:js_content_handler
+
+define("main", [
+  "console",
+  "mojo/services/public/js/service-provider",
+  "services/js/test/echo_service.mojom",
+  "services/js/app-bridge",
+], function(console, spModule, echoModule, appModule) {
+
+  function Application(shell, url) {
+    this.shell = shell;
+    this.serviceProviders = [];
+  }
+
+  Application.prototype.initialize = function(args) {
+  }
+
+  Application.prototype.acceptConnection = function(url, spHandle) {
+    function EchoServiceImpl(client) {
+      this.echoString = function(s) {
+        if (s == "quit")
+          appModule.quit();
+        return Promise.resolve({value: s});
+      };
+    }
+    var serviceProvider =  new spModule.ServiceProvider(spHandle);
+    serviceProvider.provideService(echoModule.EchoService, EchoServiceImpl);
+    this.serviceProviders.push(serviceProvider);
+  }
+
+  return Application;
+});