Make plugin loading async

Summary: This diff makes plugin loading async, which we'd need in a browser env (either because we'd use `import()` or we need to fetch the source and than eval it), and deals with all the fallout of that

Reviewed By: timur-valiev

Differential Revision: D32669995

fbshipit-source-id: 73babf38a6757c451b8200c3b320409f127b8b5b

desktop/scripts/jest-setup-after.ts

@@ -168,5 +168,8 @@ function createStubRenderHost(): RenderHost {
       return stubConfig.gatekeepers[gk] ?? false;
     },
     flipperServer: TestUtils.createFlipperServerMock(),
+    async requirePlugin(path: string) {
+      return require(path);
+    },
   };
 }