Allow plugins to use css

Summary:
Flipper plugins fail when importing css from third-party dependencies. This diff tries to fix that.

Effectively, the plugin can import the css and export it when is bundled.

When we load the plugin, we check if there's a css file for it. If there's one, we return it and try to use it.

Reviewed By: aigoncharov

Differential Revision: D40758178

fbshipit-source-id: e53afffcc481504905d5eeb1aea1f9114ee2a86b

desktop/flipper-ui-core/src/__tests__/__snapshots__/createMockFlipperWithPlugin.node.tsx.snap

@@ -57,6 +57,7 @@ exports[`can create a Fake flipper with legacy wrapper 2`] = `
 Object {
   "clientPlugins": Map {
     "TestPlugin" => SandyPluginDefinition {
+      "css": undefined,
       "details": Object {
         "dir": "/Users/mock/.flipper/thirdparty/flipper-plugin-sample1",
         "entry": "./test/index.js",

desktop/flipper-ui-core/src/dispatcher/__tests__/plugins.node.tsx

@@ -79,6 +79,7 @@ test('requirePluginInternal loads plugin', async () => {
   expect(plugin).not.toBeNull();
   expect(Object.keys(plugin as any)).toEqual([
     'id',
+    'css',
     'details',
     'isDevicePlugin',
     'module',

desktop/flipper-ui-core/src/dispatcher/plugins.tsx

@@ -131,19 +131,24 @@ export const requirePlugin = (pluginDetails: ActivatablePluginDetails) =>
 export const requirePluginInternal = async (
   pluginDetails: ActivatablePluginDetails,
 ): Promise<PluginDefinition> => {
-  let plugin = await getRenderHostInstance().requirePlugin(
+  const requiredPlugin = await getRenderHostInstance().requirePlugin(
     (pluginDetails as InstalledPluginDetails).entry,
   );
-  if (!plugin) {
+  if (!requiredPlugin || !requiredPlugin.plugin) {
     throw new Error(
       `Failed to obtain plugin source for: ${pluginDetails.name}`,
     );
   }
   if (isSandyPlugin(pluginDetails)) {
     // Sandy plugin
-    return new _SandyPluginDefinition(pluginDetails, plugin);
+    return new _SandyPluginDefinition(
+      pluginDetails,
+      requiredPlugin.plugin,
+      requiredPlugin.css,
+    );
   } else {
-    // classic plugin
+    // Classic plugin
+    let plugin = requiredPlugin.plugin;
     if (plugin.default) {
       plugin = plugin.default;
     }