Ensure tests don't depend on fake devices being added synchronously

third_party/WebKit/LayoutTests/usb/resources/usb-helpers.js

 'use strict';
 
 function usb_test(func, name, properties) {
   promise_test(() => {
     return navigator.usb.test.initialize().then(() => {
       let testResult = func();
       let cleanup = () => {
         navigator.usb.test.reset();
       };
       testResult.then(cleanup, cleanup);
       return testResult;
     });
   }, name, properties);
 }
 
+// Returns a promise that is resolved when the next USBConnectionEvent of the
+// given type is received.
+function connectionEventPromise(eventType) {
+  return new Promise(resolve => {
+    let eventHandler = e => {
+      assert_true(e instanceof USBConnectionEvent);
+      navigator.usb.removeEventListener(eventType, eventHandler);
+      resolve(e.device);
+    };
+    navigator.usb.addEventListener(eventType, eventHandler);
+  });
+}
+
+function getConnectedDevice(deviceInit) {

[ortuno, 2017/04/12 03:31:07]

Comment what the function does.

Optional: You might want to change the name of the function it's not really
obvious what it does from reading a test.

[Reilly Grant (use Gerrit), 2017/04/12 22:47:07]

Done.

+  let promise = connectionEventPromise('connect');
+  navigator.usb.test.addFakeDevice(deviceInit);
+  return promise;
+}
+
 function assertRejectsWithError(promise, name, message) {
   return promise.then(() => {
     assert_unreached('expected promise to reject with ' + name);
   }, error => {
     assert_equals(error.name, name);
     if (message !== undefined)
       assert_equals(error.message, message);
   });
 }
 
@@ skipping 30 matching lines @@
   });
 }
 
 function runGarbageCollection() {
   // Run gc() as a promise.
   return new Promise((resolve, reject) => {
     GCController.collect();
     setTimeout(resolve, 0);
   });
 }