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/flipper-ui-core/src/reducers/__tests__/sandyplugins.node.tsx

@@ -18,6 +18,7 @@ import {
   TestUtils,
 } from 'flipper-plugin';
 import {switchPlugin} from '../pluginManager';
+import {awaitPluginCommandQueueEmpty} from '../../dispatcher/pluginManager';
 
 const pluginDetails = TestUtils.createMockPluginDetails();
 
@@ -184,6 +185,8 @@ test('it should not initialize a sandy plugin if not enabled', async () => {
     }),
   );
 
+  await awaitPluginCommandQueueEmpty(store);
+
   expect(client.sandyPluginStates.get(Plugin2.id)).toBeUndefined();
   expect(instance.connectStub).toHaveBeenCalledTimes(0);
   // disconnect wasn't called because connect was never called