Fix missing `Ast` flow type

Summary: The type was not imported and this escaped the attention of Flow somehow.

Reviewed By: davidaurelio

Differential Revision: D6834298

fbshipit-source-id: 96f754d5aae9fda661cb18bbbb30ccfd033598a1

packages/metro/src/ModuleGraph/worker/optimizeDependencies.js

@@ -16,6 +16,7 @@ const {traverse} = require('babel-core');
 const prettyPrint = require('babel-generator').default;
 
 import type {TransformResultDependency} from '../types.flow';
+import type {Ast} from 'babel-core';
 
 type Context = {
   oldToNewIndex: Map<number, number>,