Fix #17610, Add fixtures to metro blacklist

Summary:
Include a default blacklist into the build settings to prevent
processing of incorrect fixture files by Metro.

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Fix #17610 issue, preventing metro from processing fixture files

1. Have a working demo
2. Install https://github.com/oblador/react-native-vector-icons
3. Use in a component
4. Start the app
5. The app starts successfully and display the icons

[ GENERAL  ]  [ BUGFIX ]  [local-cli/util/Config.js] - Add default file blacklist
Closes https://github.com/facebook/react-native/pull/17672

Differential Revision: D7014627

Pulled By: hramos

fbshipit-source-id: 20974e6fdd0977eeeb1048c29c9d621c803c26e9

local-cli/util/Config.js

@@ -15,7 +15,7 @@ const getPolyfills = require('../../rn-get-polyfills');
 const invariant = require('fbjs/lib/invariant');
 const path = require('path');
 
-const {Config: MetroConfig} = require('metro');
+const {Config: MetroConfig, createBlacklist} = require('metro');
 
 const RN_CLI_CONFIG = 'rn-cli.config.js';
 
@@ -54,6 +54,10 @@ const getProjectRoots = () => {
   return resolveSymlinksForRoots([getProjectPath()]);
 };
 
+const getBlacklistRE = () => {
+  return createBlacklist([/.*\/__fixtures__\/.*/]);
+};
+
 /**
  * Module capable of getting the configuration out of a given file.
  *
@@ -65,6 +69,7 @@ const getProjectRoots = () => {
 const Config = {
   DEFAULT: ({
     ...MetroConfig.DEFAULT,
+    getBlacklistRE,
     getProjectRoots,
     getPolyfills,
     getModulesRunBeforeMainModule: () => [