Add some Flow typs

Add some Flow types

scripts/tasks/flow.js

@@ -0,0 +1,40 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+'use strict';
+
+const {spawn} = require('child_process');
+
+async function runFlow(renderer, args) {
+  return new Promise(resolve => {
+    let cmd = __dirname + '/../../node_modules/.bin/flow';
+    if (process.platform === 'win32') {
+      cmd = cmd.replace(/\//g, '\\') + '.cmd';
+    }
+
+    console.log(
+      'Running Flow...',
+    );
+
+    spawn(cmd, args, {
+      // Allow colors to pass through:
+      stdio: 'inherit',
+    }).on('close', function(code) {
+      if (code !== 0) {
+        console.error(
+          'Flow failed :(',
+        );
+        console.log();
+        process.exit(code);
+      } else {
+        resolve();
+      }
+    });
+  });
+}
+
+runFlow();