Stop yarn error message appearing for Windows users of local-cli

Summary:
On windows, recent versions of local-cli will display a yarn error to stderr when starting the packager (see https://github.com/expo/xde/issues/91, https://github.com/react-community/create-react-native-app/issues/101, https://github.com/react-community/create-react-native-app/issues/113#issuecomment-289185491 for examples of users hitting this in the wild), even though no package management action is being taken.

From what I can tell this is what happens:

* [`local-cli/util/yarn.js` does not ignore stderr on Windows](https://github.com/facebook/react-native/blob/6fa87134fc68fd447e33a01a538ae0af6710e5d2/local-cli/util/yarn.js#L25)
* [`local-cli/util/PackageManager.js` calls the above function when it's require'd](https://github.com/facebook/react-native/blob/6fa87134fc68fd447e33a01a538ae0af6710e5d2/local-cli/util/PackageManager.js#L20)

For Windows users who don't have yarn installed, this means that the 'yarn is not recognized as an internal or external command..." error displays wh
Closes https://github.com/facebook/react-native/pull/13355

Differential Revision: D4848084

Pulled By: hramos

fbshipit-source-id: f32176354e0bd7ff6d7009ea30dca64ff23ae3d5

local-cli/util/PackageManager.js

@@ -15,11 +15,6 @@ const spawnOpts = {
   stdin: 'inherit',
 };
 
-const projectDir = process.cwd();
-const isYarnAvailable =
-  yarn.getYarnVersionIfAvailable() &&
-  yarn.isGlobalCliUsingYarn(projectDir);
-
 /**
  * Execute npm or yarn command
  *
@@ -30,6 +25,11 @@ const isYarnAvailable =
 function callYarnOrNpm(yarnCommand, npmCommand) {
   let command;
 
+  const projectDir = process.cwd();
+  const isYarnAvailable =
+    yarn.getYarnVersionIfAvailable() &&
+    yarn.isGlobalCliUsingYarn(projectDir);
+
   if (isYarnAvailable) {
     command = yarnCommand;
   } else {

local-cli/util/yarn.js

@@ -21,11 +21,9 @@ function getYarnVersionIfAvailable() {
   let yarnVersion;
   try {
     // execSync returns a Buffer -> convert to string
-    if (process.platform.startsWith('win')) {
-      yarnVersion = (execSync('yarn --version').toString() || '').trim();
-    } else {
-      yarnVersion = (execSync('yarn --version 2>/dev/null').toString() || '').trim();
-    }
+    yarnVersion = (execSync('yarn --version', {
+      stdio: [ 0, 'pipe', 'ignore', ]
+    }).toString() || '').trim();
   } catch (error) {
     return null;
   }