[js] Export promise.createPromise(), which will return a managed prom…

…ise if

the promise manager is enabled, otherwise it will return a native promise.

For #2969

javascript/node/selenium-webdriver/lib/promise.js

@@ -1561,10 +1561,11 @@ function defer() {
  * @template T
  */
 function fulfilled(opt_value) {
-  if (usePromiseManager()) {
-    return ManagedPromise.resolve(opt_value);
+  let ctor = usePromiseManager() ? ManagedPromise : NativePromise;
+  if (opt_value instanceof ctor) {
+    return /** @type {!Thenable<T>} */(opt_value);
   }
-  return NativePromise.resolve(opt_value);
+  return ctor.resolve(opt_value);
 }
 
 
@@ -2081,6 +2082,10 @@ function usePromiseManager() {
 
 
 /**
+ * Creates a new promise with the given `resolver` function. If the promise
+ * manager is currently enabled, the returned promise will be a
+ * {@linkplain ManagedPromise} instance. Otherwise, it will be a native promise.
+ *
  * @param {function(
  *             function((T|IThenable<T>|Thenable|null)=),
  *             function(*=))} resolver
@@ -3317,6 +3322,7 @@ module.exports = {
   consume: consume,
   controlFlow: controlFlow,
   createFlow: createFlow,
+  createPromise: createPromise,
   defer: defer,
   delayed: delayed,
   filter: filter,

javascript/node/selenium-webdriver/test/lib/promise_test.js

@@ -301,6 +301,25 @@ describe('promise', function() {
   });
 
   promiseManagerSuite(() => {
+    describe('fulfilled', function() {
+      it('returns input value if it is already a valid promise', function() {
+        let p = promise.createPromise(function() {});
+        let r = promise.fulfilled(p);
+        assert.strictEqual(p, r);
+      });
+
+      it('creates a new promise fulfilled with input', function() {
+        return promise.fulfilled(1234).then(v => assert.equal(1234, v));
+      });
+
+      it('can convert thenables to valid promise', function() {
+        let thenable = {then: function(cb) {cb(1234)}};
+        let p = promise.fulfilled(thenable);
+        assert.notStrictEqual(thenable, p);
+        return p.then(v => assert.equal(1234, v));
+      });
+    });
+
     describe('when', function() {
       it('ReturnsAResolvedPromiseIfGivenANonPromiseValue', function() {
         var ret = promise.when('abc');