Pass supportsMethod as an argument to the exportPersistedState

Summary:
This diff adds supportsMethod as a closure which can be used to verify a method is implemented on the client side.

It will be used in the next diff

Reviewed By: jknoxville

Differential Revision: D21176415

fbshipit-source-id: fe16d966c58d36558034ce4ade8f58f8031aab18

desktop/app/src/utils/__tests__/pluginUtils.node.js

@@ -40,6 +40,7 @@ function createMockFlipperPluginWithExportPersistedState(id: string) {
       callClient: (string, ?Object) => Promise<Object>,
       persistedState: ?{msg: string},
       store: ?ReduxState,
+      supportsMethod?: (string) => Promise<boolean>,
     ): Promise<?{msg: string}> => {
       return Promise.resolve({
         msg: 'MockFlipperPluginWithExportPersistedState',