Export 'app' in v2 entrypoint (#1615)

* Export 'app' in v2 entrypoint.

Fixes firebase/firebase-tools#7637.

* Add changelog.

* Add comment

Author: taeold

CHANGELOG.md

@@ -0,0 +1 @@
+- Fix bug where v1 functions can't be emulated (#1615)

src/v2/index.ts

@@ -77,3 +77,7 @@ export { params };
 // NOTE: Required to support the Functions Emulator which monkey patches `functions.config()`
 // TODO(danielylee): Remove in next major release.
 export { config } from "../v1/config";
+
+// Required for v1 Emulator support.
+import { setApp as setEmulatedAdminApp } from "../common/app";
+export const app = { setEmulatedAdminApp };