Fix missing `Ast` flow type

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

Reviewed By: davidaurelio

Differential Revision: D6834292

fbshipit-source-id: 8a9b5f2ac56fca2d0ff298b52f54935327eb2686

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

@@ -19,6 +19,7 @@ const {traverse, types} = require('babel-core');
 const prettyPrint = require('babel-generator').default;
 
 import type {TransformResultDependency} from '../types.flow';
+import type {Ast} from 'babel-core';
 
 export type DynamicRequiresBehavior = 'throwAtRuntime' | 'reject';
 type Options = {|