small refactoring to prepare async work

web/static/tests/helpers.ts

@@ -199,3 +199,19 @@ export function nextMicroTick(): Promise<void> {
 export function nextTick(): Promise<void> {
   return new Promise(resolve => setTimeout(resolve));
 }
+
+interface Deferred extends Promise<any> {
+  resolve(val?: any): void;
+  reject(): void;
+}
+
+export function makeDeferred(): Deferred {
+  let resolve, reject;
+  let def = new Promise((_resolve, _reject) => {
+    resolve = _resolve;
+    reject = _reject;
+  });
+  (<Deferred>def).resolve = resolve;
+  (<Deferred>def).reject = reject;
+  return <Deferred>def;
+}