Implement binding support for returning and accepting Promises in WebIDL.

components/script/dom/webidls/Promise.webidl

@@ -0,0 +1,11 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// This interface is entirely internal to Servo, and should not be accessible to
+// web pages.
+
+[NoInterfaceObject]
+// Need to escape "Promise" so it's treated as an identifier.
+interface _Promise {
+};

components/script/dom/webidls/TestBinding.webidl

@@ -508,6 +508,11 @@ interface TestBinding {
   [Func="TestBinding::condition_satisfied"]
   const unsigned short funcControlledConstEnabled = 0;
 
+  Promise<DOMString> returnPromise();
+  readonly attribute Promise<boolean> promiseAttribute;
+  void acceptPromise(Promise<DOMString> string);
+  void acceptNullablePromise(Promise<DOMString>? string);
+
   void panic();
 };