Rename `Ast` to a more appropriate `BabelNode` in the babel flow type

Summary:
An Ast consists of `Node` elements and the one Babel uses is specific to Babel (incompatible with Astree spec by default) so `BabelNode` seems more appropriate.

Keeping the other `ast` names for now to reduce the footprint of this diff.

Also makes sure `babel-core` properly exports the same node type as what the api returns making fixes to this type in the future easier to implement.

Reviewed By: davidaurelio

Differential Revision: D6834291

fbshipit-source-id: 4cb0eb2ee280a4988071b8412539edfad12cf56f

flow-typed/babel.js.flow

@@ -102,10 +102,10 @@ type _TransformOptions = __TransformOptions & {
   env?: {[key: string]: __TransformOptions},
 };
 
-type Ast = {};
+type BabelNode = {};
 
 type TransformResult = {
-  ast: Ast,
+  ast: BabelNode,
   code: ?string,
   ignored: boolean,
   map: ?_BabelSourceMap,
@@ -123,14 +123,13 @@ type BabelTypes = {[key: string]: Function};
 declare module 'babel-core' {
   declare type Plugins = _Plugins;
   declare type BabelSourceMap = _BabelSourceMap;
-  declare type Ast = {};
+  declare type Ast = BabelNode;
   declare type TransformOptions = _TransformOptions;
   declare var transform: TransformSync /*(
     code: string,
     options?: _TransformOptions,
   ): TransformResult*/;
   declare function traverse<State>(
-    ast: Ast,
     ast: BabelNode,
     visitor: BabelVisitor,
     scope?: ?Object,
@@ -140,7 +139,7 @@ declare module 'babel-core' {
   declare var types: BabelTypes;
   declare var transformFile: TransformFile;
   declare var transformFromAst: TransformFromAst /*(
-    ast: Ast,
+    ast: BabelNode,
     code?: ?string,
     babelOptions?: _TransformOptions,
   ): TransformResult*/;
@@ -157,7 +156,7 @@ type _BabelSourceMapSegment = {
 declare module 'babel-generator' {
   declare export type BabelSourceMapSegment = _BabelSourceMapSegment;
   declare export default (
-    ast: Ast,
+    ast: BabelNode,
     options?: GeneratorOptions,
     code?: string | {[string]: string},
   ) => GeneratorResult

packages/metro/src/Bundler/util.js

@@ -17,6 +17,7 @@ const {babelTypes} = require('../babel-bridge');
 
 import type {AssetDataWithoutFiles} from '../Assets';
 import type {ModuleTransportLike} from '../shared/types.flow';
+import type {Ast} from '@babel/core';
 
 // Structure of the object: dir.name.scale = asset
 export type RemoteFileMap = {