added to remote repo

frontend/node_modules/agent-base/src/promisify.ts

@@ -0,0 +1,33 @@
+import {
+	Agent,
+	ClientRequest,
+	RequestOptions,
+	AgentCallbackCallback,
+	AgentCallbackPromise,
+	AgentCallbackReturn
+} from './index';
+
+type LegacyCallback = (
+	req: ClientRequest,
+	opts: RequestOptions,
+	fn: AgentCallbackCallback
+) => void;
+
+export default function promisify(fn: LegacyCallback): AgentCallbackPromise {
+	return function(this: Agent, req: ClientRequest, opts: RequestOptions) {
+		return new Promise((resolve, reject) => {
+			fn.call(
+				this,
+				req,
+				opts,
+				(err: Error | null | undefined, rtn?: AgentCallbackReturn) => {
+					if (err) {
+						reject(err);
+					} else {
+						resolve(rtn);
+					}
+				}
+			);
+		});
+	};
+}