Use constants from flipper-frontend-core in flipper-ui-core

Summary: See D37139129

Reviewed By: lblasa

Differential Revision: D37238570

fbshipit-source-id: 5dbf6f179fc8957e074bad1e8f74e400644be049

desktop/flipper-frontend-core/src/index.tsx

@@ -22,3 +22,4 @@ export * from './plugins';
 export {getPluginKey} from './utils/pluginKey';
 export * from './flipperLibImplementation';
 export * from './client/FlipperServerClient';
+export {default as frontendCoreConstants} from './fb-stubs/constants';