Use sync fs.mkdir

Summary:
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This ensures that errors are actually noticed in case it is not
possible to create the directory. This will also throw an error when used with the upcoming Node.js 10.x because there is no callback.

Refs: https://github.com/nodejs/node/pull/18668

[CLI][MINOR][local-cli/library/library.js] - Use sync fs.mkdir for error handling
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  CATEGORY
[----------]        TYPE
[ CLI      ]   [-------------]      LOCATION
[ DOCS     ]   [ BREAKING    ]   [-------------]
[ GENERAL  ]   [ BUGFIX      ]   [-{Component}-]
[ INTERNAL ]   [ ENHANCEMENT ]   [ {File}      ]
[ IOS      ]   [ FEATURE     ]   [ {Directory} ]   |-----------|
[ ANDROID  ]   [ MINOR       ]   [ {Framework} ] - | {Message} |
[----------]   [-------------]   [-------------]   |-----------|

[CATEGORY] [TYPE] [LOCATION] - MESSAGE

 EXAMPLES:

 [IOS] [BREAKING] [FlatList] - Change a thing that breaks other things
 [ANDROID] [BUGFIX] [TextInput] - Did a thing to TextInput
 [CLI] [FEATURE] [local-cli/info/info.js] - CLI easier to do things with
 [DOCS] [BUGFIX] [GettingStarted.md] - Accidentally a thing/word
 [GENERAL] [ENHANCEMENT] [Yoga] - Added new yoga thing/position
 [INTERNAL] [FEATURE] [./scripts] - Added thing to script that nobody will see
-->
Closes https://github.com/facebook/react-native/pull/18084

Differential Revision: D7083122

Pulled By: shergin

fbshipit-source-id: a13b64e57860e1ab2c15e0c008211dc0dca3b2d2

local-cli/library/library.js

@@ -29,7 +29,7 @@ function library(argv, config, args) {
   const source = path.resolve('node_modules', 'react-native', 'Libraries', 'Sample');
 
   if (!fs.existsSync(libraries)) {
-    fs.mkdir(libraries);
+    fs.mkdirSync(libraries);
   }
 
   if (fs.existsSync(libraryDest)) {