Fix failure due to missing Metro flow types

Summary:
Fixes the Flow failure due to an undefined Ast type.

Before:

```
 $ npm run flow -- check

> [email protected] flow /Users/hramos/git/react-native
> flow "check"

Error: local-cli/__tests__/fs-mock-test.js:27
 27:   beforeEach(() => {
       ^^^^^^^^^^ beforeEach. Could not resolve name

Error: local-cli/__tests__/fs-mock-test.js:53
 53:       expect(content).toEqual('beep');
           ^^^^^^ expect. Could not resolve name

Error: local-cli/__tests__/fs-mock-test.js:88
 88:         expect(content).toEqual('hello, world!');
             ^^^^^^ expect. Could not resolve name

Error: local-cli/__tests__/fs-mock-test.js:100
100:       expect(content).toEqual('hello, world!');
           ^^^^^^ expect. Could not resolve name

Error: node_modules/metro/src/Bundler/util.js.flow:46
 46: ): Ast {
        ^^^ Ast. Could not resolve name

Error: node_modules/metro/src/ModuleGraph/worker/collect-dependencies.js.flow:283
283: const xp = (module.exports = (ast: Ast) =>
                                        ^^^ Ast. Could not resolve name

Error: node_modules/metro/src/assetTransformer.js.flow:29
 29: ): Promise<{ast: Ast}> {
                      ^^^ Ast. Could not resolve name
```

After

```
$ npm run flow -- check

> [email protected] flow /Users/hramos/git/react-native
> flow "check"

Error: local-cli/__tests__/fs-mock-test.js:27
 27:   beforeEach(() => {
       ^^^^^^^^^^ beforeEach. Could not resolve name

Error: local-cli/__tests__/fs-mock-test.js:53
 53:       expect(content).toEqual('beep');
           ^^^^^^ expect. Could not resolve name

Error: local-cli/__tests__/fs-mock-test.js:88
 88:         expect(content).toEqual('hello, world!');
             ^^^^^^ expect. Could not resolve name

Error: local-cli/__tests__/fs-mock-test.js:100
100:       expect(content).toEqual('hello, world!');
           ^^^^^^ expect. Could not resolve name
```

[ GENERAL ] [ BUGFIX] [ .flowconfig ] - Have Flow ignore Metro node_nodules
Closes #17187

Differential Revision: D6572303

Pulled By: hramos

fbshipit-source-id: aa256b9725970fcc2a6da6578c83e7c0875e3cfd

Author: hramos

Diff for: .flowconfig

@@ -22,11 +22,15 @@
 ; Ignore polyfills
 .*/Libraries/polyfills/.*
 
+; Ignore metro
+.*/node_modules/metro/.*
+
 [include]
 
 [libs]
 Libraries/react-native/react-native-interface.js
 flow/
+flow-github/
 
 [options]
 emoji=true

Diff for: flow-github/metro.js

@@ -0,0 +1,22 @@
+/**
+ * Copyright (c) 2017-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @flow
+ */
+
+declare module 'metro' {
+  declare module.exports: any;
+}
+
+declare module 'metro/src/lib/TerminalReporter' {
+  declare module.exports: any;
+}
+
+declare module 'metro/src/HmrServer' {
+  declare module.exports: any;
+}

Diff for: local-cli/templates/HelloWorld/_flowconfig

@@ -16,11 +16,15 @@
 ; Ignore polyfills
 .*/Libraries/polyfills/.*
 
+; Ignore metro
+.*/node_modules/metro/.*
+
 [include]
 
 [libs]
 node_modules/react-native/Libraries/react-native/react-native-interface.js
 node_modules/react-native/flow/
+node_modules/react-native/flow-github/
 
 [options]
 emoji=true