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-server-core/src/plugins/PluginManager.tsx

@@ -61,6 +61,10 @@ export class PluginManager {
   installPluginFromFile = installPluginFromFile;
   installPluginFromNpm = installPluginFromNpm;
 
+  async loadSource(path: string) {
+    return await fs.readFile(path, 'utf8');
+  }
+
   async getBundledPlugins(): Promise<Array<BundledPluginDetails>> {
     if (process.env.NODE_ENV === 'test') {
       return [];